Add data-URI support class to dart:core (next to Uri).

sdk/lib/core/uri.dart

Index: sdk/lib/core/uri.dart
diff --git a/sdk/lib/core/uri.dart b/sdk/lib/core/uri.dart
index 33f5faf33953c68232c5b1b215d1dfe83864a0d5..2757c05f98bb652bf561cc0c5d8c52adec9b4df6 100644
--- a/sdk/lib/core/uri.dart
+++ b/sdk/lib/core/uri.dart
@@ -745,6 +745,72 @@ class Uri {
   }
 
   /**
+   * Creates a `data:` URI containing the [content] string.
+   *
+   * Converts the content to a bytes using [encoding] or the charset specified
+   * in [parameters] (defaulting to US-ASCII if not specified or unrecognized),
+   * then encodes the bytes into the resulting data URI.
+   *
+   * Defaults to encoding using percent-encoding (any non-ASCII or non-URI-valid
+   * bytes is replaced by a percent encoding). If [base64] is true, the bytes
+   * are instead encoded using [BASE64].
+   *
+   * If [encoding] is not provided and [parameters] has a `charset` entry,
+   * that name is looked up using [Encoding.getByName],
+   * and if the lookup returns an encoding, that encoding is used to convert
+   * [content] to bytes.
+   * If providing both an [encoding] and a charset [parameter], they should
+   * agree, otherwise decoding won't be able to use the charset parameter
+   * to determine the encoding.
+   *
+   * If [mimeType] and/or [parameters] are supplied, they are added to the
+   * created URI. If any of these contain characters that are not allowed
+   * in the data URI, the character is percent-escaped. If the character is
+   * non-ASCII, it is first UTF-8 encoded and then the bytes are percent
+   * encoded. An omitted [mimeType] in a data URI means `text/plain`, just
+   * as an omitted `charset` parameter defaults to meaning `US-ASCII`.
+   *
+   * To read the content back, use [UriData.contentAsString].
+   */
+  factory Uri.dataFromString(String content,
+                             {String mimeType,
+                              Encoding encoding,
+                              Map<String, String> parameters,
+                              bool base64: false}) {
+    return new UriData.fromString(content,
+                                  mimeType: mimeType,
+                                  encoding: encoding,
+                                  parameters: parameters,
+                                  base64: base64).uri;

[floitsch, 2015/11/07 02:56:26]

move ".uri" to the next line, so it's visible.

[Lasse Reichstein Nielsen, 2015/11/09 10:27:38]

Done.

+  }
+
+  /**
+   * Creates a `data:` URI containing an encoding of [bytes].
+   *
+   * Defaults to Base64 encoding the bytes, but if [percentEncoded]
+   * is `true`, the bytes will instead be percent encoded (any non-ASCII
+   * or non-valid-ASCII-character byte is replaced by a percent encoding).
+   *
+   * To read the bytes back, use [UriData.contentAsBytes].
+   *
+   * It defaults to having the mime-type `application/octet-stream`.
+   * The [mimeType] and [parameters] are added to the created URI.
+   * If any of these contain characters that are not allowed
+   * in the data URI, the character is percent-escaped. If the character is
+   * non-ASCII, it is first UTF-8 encoded and then the bytes are percent
+   * encoded.
+   */
+  factory Uri.dataFromBytes(List<int> bytes,
+                            {mimeType: "application/octet-stream",
+                             Map<String, String> parameters,
+                             percentEncoded: false}) {
+    return new UriData.fromBytes(bytes,
+                                 mimeType: mimeType,
+                                 parameters: parameters,
+                                 percentEncoded: percentEncoded).uri;

[floitsch, 2015/11/07 02:56:27]

ditto.

[Lasse Reichstein Nielsen, 2015/11/09 10:27:38]

Done.

+  }
+
+  /**
    * Returns the natural base URI for the current platform.
    *
    * When running in a browser this is the current URL (from
@@ -1298,19 +1364,6 @@ class Uri {
 
   static int _stringOrNullLength(String s) => (s == null) ? 0 : s.length;
 
-  static bool _isHexDigit(int char) {
-    if (_NINE >= char) return _ZERO <= char;
-    char |= 0x20;
-    return _LOWER_CASE_A <= char && _LOWER_CASE_F >= char;
-  }
-
-  static int _hexValue(int char) {
-    assert(_isHexDigit(char));
-    if (_NINE >= char) return char - _ZERO;
-    char |= 0x20;
-    return char - (_LOWER_CASE_A - 10);
-  }
-
   /**
    * Performs RFC 3986 Percent-Encoding Normalization.
    *
@@ -1331,10 +1384,12 @@ class Uri {
     }
     int firstDigit = source.codeUnitAt(index + 1);
     int secondDigit = source.codeUnitAt(index + 2);
-    if (!_isHexDigit(firstDigit) || !_isHexDigit(secondDigit)) {
+    int firstDigitValue = _parseHexDigit(firstDigit);
+    int secondDigitValue = _parseHexDigit(secondDigit);
+    if (firstDigitValue < 0 || secondDigitValue < 0) {
       return "%";  // Marks the escape as invalid.
     }
-    int value = _hexValue(firstDigit) * 16 + _hexValue(secondDigit);
+    int value = firstDigitValue * 16 + secondDigitValue;
     if (_isUnreservedChar(value)) {
       if (lowerCase && _UPPER_CASE_A <= value && _UPPER_CASE_Z >= value) {
         value |= 0x20;
@@ -1350,21 +1405,32 @@ class Uri {
     return null;
   }
 
-  static bool _isUnreservedChar(int ch) {
-    return ch < 127 &&
-           ((_unreservedTable[ch >> 4] & (1 << (ch & 0x0f))) != 0);
+  // Converts a UTF-16 code-unit to its value as a hex digit.
+  // Returns -1 for non-hex digits.
+  static int _parseHexDigit(int char) {
+    int digit = char ^ Uri._ZERO;
+    if (digit <= 9) return digit;
+    int lowerCase = char | 0x20;
+    if (Uri._LOWER_CASE_A <= lowerCase && lowerCase <= _LOWER_CASE_F) {
+      return lowerCase - (_LOWER_CASE_A - 10);
+    }
+    return -1;
   }
 
-  static String _escapeChar(char) {
+  static bool _isUnreservedChar(int char) {

[floitsch, 2015/11/07 02:56:27]

Move that function closer to the table.

[Lasse Reichstein Nielsen, 2015/11/09 10:27:38]

Done.

+    return char < 127 &&
+           ((_unreservedTable[char >> 4] & (1 << (char & 0x0f))) != 0);
+  }
+
+  static String _escapeChar(int char) {
     assert(char <= 0x10ffff);  // It's a valid unicode code point.
-    const hexDigits = "0123456789ABCDEF";
     List codeUnits;
     if (char < 0x80) {
       // ASCII, a single percent encoded sequence.
       codeUnits = new List(3);
       codeUnits[0] = _PERCENT;
-      codeUnits[1] = hexDigits.codeUnitAt(char >> 4);
-      codeUnits[2] = hexDigits.codeUnitAt(char & 0xf);
+      codeUnits[1] = _hexDigits.codeUnitAt(char >> 4);
+      codeUnits[2] = _hexDigits.codeUnitAt(char & 0xf);
     } else {
       // Do UTF-8 encoding of character, then percent encode bytes.
       int flag = 0xc0;  // The high-bit markers on the first byte of UTF-8.
@@ -1382,8 +1448,8 @@ class Uri {
       while (--encodedBytes >= 0) {
         int byte = ((char >> (6 * encodedBytes)) & 0x3f) | flag;
         codeUnits[index] = _PERCENT;
-        codeUnits[index + 1] = hexDigits.codeUnitAt(byte >> 4);
-        codeUnits[index + 2] = hexDigits.codeUnitAt(byte & 0xf);
+        codeUnits[index + 1] = _hexDigits.codeUnitAt(byte >> 4);
+        codeUnits[index + 2] = _hexDigits.codeUnitAt(byte & 0xf);
         index += 3;
         flag = 0x80;  // Following bytes have only high bit set.
       }
@@ -1886,6 +1952,16 @@ class Uri {
     }
   }
 
+  /**
+   * Access the structure of a `data:` URI.
+   *
+   * Returns a [UriData] object for `data:` URIs and `null` for all other
+   * URIs.
+   * The [UriData] object can be used to access the media type and data
+   * of a `data:` URI.
+   */
+  UriData get data => (scheme == "data") ? new UriData.fromUri(this) : null;
+
   String toString() {
     StringBuffer sb = new StringBuffer();
     _addIfNonEmpty(sb, scheme, scheme, ':');
@@ -1904,16 +1980,16 @@ class Uri {
   bool operator==(other) {
     if (other is! Uri) return false;
     Uri uri = other;
-    return scheme == uri.scheme &&
-        hasAuthority == uri.hasAuthority &&
-        userInfo == uri.userInfo &&
-        host == uri.host &&
-        port == uri.port &&
-        path == uri.path &&
-        hasQuery == uri.hasQuery &&
-        query == uri.query &&
-        hasFragment == uri.hasFragment &&
-        fragment == uri.fragment;
+    return scheme       == uri.scheme       &&
+           hasAuthority == uri.hasAuthority &&
+           userInfo     == uri.userInfo     &&
+           host         == uri.host         &&
+           port         == uri.port         &&
+           path         == uri.path         &&
+           hasQuery     == uri.hasQuery     &&
+           query        == uri.query        &&
+           hasFragment  == uri.hasFragment  &&
+           fragment     == uri.fragment;
   }
 
   int get hashCode {
@@ -2252,6 +2328,8 @@ class Uri {
   static const int _LOWER_CASE_Z = 0x7A;
   static const int _BAR = 0x7C;
 
+  static const String _hexDigits = "0123456789ABCDEF";
+
   external static String _uriEncode(List<int> canonicalTable,
                                     String text,
                                     Encoding encoding,
@@ -2294,23 +2372,26 @@ class Uri {
    */
   static String _uriDecode(String text,
                            {bool plusToSpace: false,
-                            Encoding encoding: UTF8}) {
+                            Encoding encoding: UTF8,
+                            int start: 0,
+                            int end}) {
+    if (end == null) end = text.length;

[floitsch, 2015/11/07 02:56:27]

Nit: end ??= text.length;

[Lasse Reichstein Nielsen, 2015/11/09 10:27:38]

Removed completely.

     // First check whether there is any characters which need special handling.
     bool simple = true;
-    for (int i = 0; i < text.length && simple; i++) {
+    for (int i = start; i < end && simple; i++) {
       var codeUnit = text.codeUnitAt(i);
       simple = codeUnit != _PERCENT && codeUnit != _PLUS;
     }
     List<int> bytes;
     if (simple) {
       if (encoding == UTF8 || encoding == LATIN1) {
-        return text;
+        return text.substring(start, end);
       } else {
-        bytes = text.codeUnits;
+        bytes = text.substring(start, end).codeUnits;
       }
     } else {
       bytes = new List();
-      for (int i = 0; i < text.length; i++) {
+      for (int i = start; i < end; i++) {
         var codeUnit = text.codeUnitAt(i);
         if (codeUnit > 127) {
           throw new ArgumentError("Illegal percent encoding in URI");
@@ -2332,8 +2413,8 @@ class Uri {
   }
 
   static bool _isAlphabeticCharacter(int codeUnit)
-    => (codeUnit >= _LOWER_CASE_A && codeUnit <= _LOWER_CASE_Z) ||
-       (codeUnit >= _UPPER_CASE_A && codeUnit <= _UPPER_CASE_Z);
+      => (codeUnit >= _LOWER_CASE_A && codeUnit <= _LOWER_CASE_Z) ||
+         (codeUnit >= _UPPER_CASE_A && codeUnit <= _UPPER_CASE_Z);
 
   // Tables of char-codes organized as a bit vector of 128 bits where
   // each bit indicate whether a character code on the 0-127 needs to
@@ -2582,4 +2663,594 @@ class Uri {
       0xfffe,   // 0x60 - 0x6f  0111111111111111
                 //              pqrstuvwxyz   ~
       0x47ff];  // 0x70 - 0x7f  1111111111100010
+
+}
+
+// --------------------------------------------------------------------
+// Data URI
+// --------------------------------------------------------------------
+
+/**
+ * A way to access the structure of a `data:` URI.
+ *
+ * Data URIs are non-hierarchical URIs that can contain any binary data.
+ * They are defined by [RFC 2397](https://tools.ietf.org/html/rfc2397).
+ *
+ * This class allows parsing the URI text and extracting individual parts of the
+ * URI, as well as building the URI text from structured parts.
+ */
+class UriData {
+  static const int _noScheme = -1;
+  /**
+   * Contains the text content of a `data:` URI, with or without a
+   * leading `data:`.
+   *
+   * If [_separatorIndices] starts with `4` (the index of the `:`), then
+   * there is a leading `data:`, otherwise [_separatorIndices] starts with
+   * `-1`.
+   */
+  final String _text;
+
+  /**
+   * List of the separators (';', '=' and ',') in the text.
+   *
+   * Starts with the index of the `:` in `data:` of the mimeType.
+   * That is always either -1 or 4, depending on whether `_text` includes the
+   * `data:` scheme or not.
+   *
+   * The first speparator ends the mime type. We don't bother with finding
+   * the '/' inside the mime type.
+   *
+   * Each two separators after that marks a parameter key and value.
+   *
+   * If there is a single separator left, it ends the "base64" marker.
+   *
+   * So the following separators are found for a text:
+   *
+   *     data:text/plain;foo=bar;base64,ARGLEBARGLE=
+   *         ^          ^   ^   ^      ^
+   *
+   */
+  final List<int> _separatorIndices;
+
+  /**
+   * Cache of the result returned by [uri].
+   */
+  Uri _uriCache;
+
+  UriData._(this._text, this._separatorIndices, this._uriCache);
+
+  /**
+   * Creates a `data:` URI containing the [content] string.
+   *
+   * Equivalent to `new Uri.dataFromString(...).data`, but may
+   * be more efficient if the [uri] itself isn't used.
+   */
+  factory UriData.fromString(String content,
+                             {String mimeType,
+                              Encoding encoding,
+                              Map<String, String> parameters,
+                              bool base64: false}) {
+    StringBuffer buffer = new StringBuffer();
+    List indices = [_noScheme];
+    String charsetName;
+    String encodingName;
+    if (parameters != null) charsetName = parameters["charset"];
+    if (encoding == null) {
+      if (charsetName != null) {
+        encoding = Encoding.getByName(charsetName);
+      }
+    } else if (charsetName == null) {
+      // Non-null only if parameters does not contain "charset".
+      encodingName = encoding.name;
+    }
+    encoding ??= ASCII;
+    _writeUri(mimeType, encodingName, parameters, buffer, indices);
+    indices.add(buffer.length);
+    if (base64) {
+      buffer.write(';base64,');
+      indices.add(buffer.length - 1);
+      buffer.write(encoding.fuse(BASE64).encode(content));
+    } else {
+      buffer.write(',');
+      _uriEncodeBytes(_uricTable, encoding.encode(content), buffer);
+    }
+    return new UriData._(buffer.toString(), indices, null);
+  }
+
+  /**
+   * Creates a `data:` URI containing an encoding of [bytes].
+   *
+   * Equivalent to `new Uri.dataFromBytes(...).data`, but may
+   * be more efficient if the [uri] itself isn't used.
+   */
+  factory UriData.fromBytes(List<int> bytes,
+                            {mimeType: "application/octet-stream",
+                             Map<String, String> parameters,
+                             percentEncoded: false}) {
+    StringBuffer buffer = new StringBuffer();
+    List indices = [_noScheme];
+    _writeUri(mimeType, null, parameters, buffer, indices);
+    indices.add(buffer.length);
+    if (percentEncoded) {
+      buffer.write(',');
+      _uriEncodeBytes(_uricTable, bytes, buffer);
+    } else {
+      buffer.write(';base64,');
+      indices.add(buffer.length - 1);
+      BASE64.encoder
+            .startChunkedConversion(
+                new StringConversionSink.fromStringSink(buffer))
+            .addSlice(bytes, 0, bytes.length, true);
+    }
+
+    return new UriData._(buffer.toString(), indices, null);
+  }
+
+  /**
+   * Creates a `DataUri` from a [Uri] which must have `data` as [Uri.scheme].
+   *
+   * The [uri] must have scheme `data` and no authority or fragment,
+   * and the path (concatenated with the query, if there is one) must be valid
+   * as data URI content with the same rules as [parse].
+   */
+  factory UriData.fromUri(Uri uri) {
+    if (uri.scheme != "data") {
+      throw new ArgumentError.value(uri, "uri",
+                                    "Scheme must be 'data'");
+    }
+    if (uri.hasAuthority) {
+      throw new ArgumentError.value(uri, "uri",
+                                    "Data uri must not have authority");
+    }
+    if (uri.hasFragment) {
+      throw new ArgumentError.value(uri, "uri",
+                                    "Data uri must not have a fragment part");
+    }
+    if (!uri.hasQuery) {
+      return _parse(uri.path, 0, uri);
+    }
+    // Includes path and query (and leading "data:").
+    return _parse("$uri", 5, uri);
+  }
+
+  /**
+   * Writes the initial part of a `data:` uri, from after the "data:"
+   * until just before the ',' before the data, or before a `;base64,`
+   * marker.
+   *
+   * Of an [indices] list is passed, separator indices are stored in that
+   * list.
+   */
+  static void _writeUri(String mimeType,
+                        String charsetName,
+                        Map<String, String> parameters,
+                        StringBuffer buffer, List indices) {
+    if (mimeType == null || mimeType == "text/plain") {
+      mimeType = "";
+    }
+    if (mimeType.isEmpty || identical(mimeType, "application/octet-stream")) {
+      buffer.write(mimeType);  // Common cases need no escaping.
+    } else {
+      int slashIndex = _validateMimeType(mimeType);
+      if (slashIndex < 0) {
+        throw new ArgumentError.value(mimeType, "mimeType",
+                                      "Invalid MIME type");
+      }
+      buffer.write(Uri._uriEncode(_tokenCharTable,
+                                  mimeType.substring(0, slashIndex),
+                                  UTF8, false));
+      buffer.write("/");
+      buffer.write(Uri._uriEncode(_tokenCharTable,
+                                  mimeType.substring(slashIndex + 1),
+                                  UTF8, false));
+    }
+    if (charsetName != null) {
+      if (indices != null) {
+        indices..add(buffer.length)
+               ..add(buffer.length + 8);
+      }
+      buffer.write(";charset=");
+      buffer.write(Uri._uriEncode(_tokenCharTable, charsetName, UTF8, false));
+    }
+    parameters?.forEach((var key, var value) {
+      if (key.isEmpty) {
+        throw new ArgumentError.value("", "Parameter names must not be empty");
+      }
+      if (value.isEmpty) {
+        throw new ArgumentError.value("", "Parameter values must not be empty",
+                                      'parameters["$key"]');
+      }
+      if (indices != null) indices.add(buffer.length);
+      buffer.write(';');
+      // Encode any non-RFC2045-token character and both '%' and '#'.
+      buffer.write(Uri._uriEncode(_tokenCharTable, key, UTF8, false));
+      if (indices != null) indices.add(buffer.length);
+      buffer.write('=');
+      buffer.write(Uri._uriEncode(_tokenCharTable, value, UTF8, false));
+    });
+  }
+
+  /**
+   * Checks mimeType is valid-ish (`token '/' token`).
+   *
+   * Returns the index of the slash, or -1 if the mime type is not
+   * considered valid.
+   *
+   * Currently only looks for slashes, all other characters will be
+   * percent-encoded as UTF-8 if necessary.
+   */
+  static int _validateMimeType(String mimeType) {
+    int slashIndex = -1;
+    for (int i = 0; i < mimeType.length; i++) {
+      var char = mimeType.codeUnitAt(i);
+      if (char != Uri._SLASH) continue;
+      if (slashIndex < 0) {
+        slashIndex = i;
+        continue;
+      }
+      return -1;
+    }
+    return slashIndex;
+  }
+
+  /**
+   * Parses a string as a `data` URI.
+   *
+   * The string must have the format:
+   *
+   * ```
+   * 'data:' (type '/' subtype)? (';' attribute '=' value)* (';base64')? ',' data
+   * ````
+   *
+   * where `type`, `subtype`, `attribute` and `value` are specified in RFC-2045,
+   * and `data` is a sequnce of URI-characters (RFC-2396 `uric`).
+   *
+   * This means that all the characters must be ASCII, but the URI may contain
+   * percent-escapes for non-ASCII byte values that need an interpretation
+   * to be converted to the corresponding string.
+   *
+   * Parsing doesn't check the validity of any part, it just checks that the
+   * input has the correct structure with the correct sequence of `/`, `;`, `=`
+   * and `,` delimiters.
+   *
+   * Accessing the individual parts may fail later if they turn out to have
+   * content that can't be decoded sucessfully as a string.
+   */
+  static UriData parse(String uri) {
+    if (!uri.startsWith("data:")) {
+      throw new FormatException("Does not start with 'data:'", uri, 0);
+    }
+    return _parse(uri, 5, null);
+  }
+
+  /**
+   * The [Uri] that this `UriData` is giving access to..

[floitsch, 2015/11/07 02:56:26]

Remove ".'

[Lasse Reichstein Nielsen, 2015/11/09 10:27:38]

Done.

+   *
+   * Returns a `Uri` with scheme `data` and the remainder of the data URI
+   * as path.
+   */
+  Uri get uri {
+    if (_uriCache != null) return _uriCache;
+    String path = _text;
+    String query = null;
+    int colonIndex = _separatorIndices[0];
+    int queryIndex = _text.indexOf('?', colonIndex + 1);
+    int end = null;
+    if (queryIndex >= 0) {
+      query = _text.substring(queryIndex + 1);
+      end = queryIndex;
+    }
+    path = _text.substring(colonIndex + 1, end);
+    // TODO(lrn): This is probably too simple. We should ensure URI
+    // normalization before passing in the raw strings, maybe using
+    // Uri._makePath, Uri._makeQuery.
+    _uriCache = new Uri._internal("data", "", null, null, path, query, null);
+    return _uriCache;
+  }
+
+  /**
+   * The MIME type of the data URI.
+   *
+   * A data URI consists of a "media type" followed by data.
+   * The media type starts with a MIME type and can be followed by
+   * extra parameters.
+   *
+   * Example:
+   *
+   *     data:text/plain;charset=utf-8,Hello%20World!
+   *
+   * This data URI has the media type `text/plain;charset=utf-8`, which is the
+   * MIME type `text/plain` with the parameter `charset` with value `utf-8`.
+   * See [RFC 2045](https://tools.ietf.org/html/rfc2045) for more detail.
+   *
+   * If the first part of the data URI is empty, it defaults to `text/plain`.
+   */
+  String get mimeType {
+    int start = _separatorIndices[0] + 1;
+    int end = _separatorIndices[1];
+    if (start == end) return "text/plain";
+    return Uri._uriDecode(_text, start: start, end: end);
+  }
+
+  /**
+   * The charset parameter of the media type.
+   *
+   * If the parameters of the media type contains a `charset` parameter
+   * then this returns its value, otherwise it returns `US-ASCII`,
+   * which is the default charset for data URIs.
+   */
+  String get charset {
+    int parameterStart = 1;
+    int parameterEnd = _separatorIndices.length - 1;  // The ',' before data.
+    if (isBase64) {
+      // There is a ";base64" separator, so subtract one for that as well.
+      parameterEnd -= 1;
+    }
+    for (int i = parameterStart; i < parameterEnd; i += 2) {
+      var keyStart = _separatorIndices[i] + 1;
+      var keyEnd = _separatorIndices[i + 1];
+      if (keyEnd == keyStart + 7 && _text.startsWith("charset", keyStart)) {
+        return Uri._uriDecode(_text, start: keyEnd + 1,
+                                     end: _separatorIndices[i + 2]);
+      }
+    }
+    return "US-ASCII";
+  }
+
+  /**
+   * Whether the data is Base64 encoded or not.
+   */
+  bool get isBase64 => _separatorIndices.length.isOdd;
+
+  /**
+   * The content part of the data URI, as its actual representation.
+   *
+   * This string may contain percent escapes.
+   */
+  String get contentText => _text.substring(_separatorIndices.last + 1);
+
+  /**
+   * The content part of the data URI as bytes.
+   *
+   * If the data is Base64 encoded, it will be decoded to bytes.
+   *
+   * If the data is not Base64 encoded, it will be decoded by unescaping
+   * percent-escaped characters and returning byte values of each unescaped
+   * character. The bytes will not be, e.g., UTF-8 decoded.
+   */
+  List<int> contentAsBytes() {
+    String text = _text;
+    int start = _separatorIndices.last + 1;
+    if (isBase64) {
+      return BASE64.decoder.convert(text, start);
+    }
+
+    // Not base64, do percent-decoding and return the remaining bytes.
+    // Compute result size.
+    const int percent = 0x25;
+    int length = text.length - start;
+    for (int i = start; i < text.length; i++) {
+      var codeUnit = text.codeUnitAt(i);
+      if (codeUnit == percent) {
+        i += 2;
+        length -= 2;
+      }
+    }
+    // Fill result array.
+    Uint8List result = new Uint8List(length);
+    if (length == text.length) {
+      result.setRange(0, length, text.codeUnits, start);
+      return result;
+    }
+    int index = 0;
+    for (int i = start; i < text.length; i++) {
+      var codeUnit = text.codeUnitAt(i);
+      if (codeUnit != percent) {
+        result[index++] = codeUnit;
+      } else {
+        if (i + 2 < text.length) {
+          var digit1 = Uri._parseHexDigit(text.codeUnitAt(i + 1));
+          var digit2 = Uri._parseHexDigit(text.codeUnitAt(i + 2));
+          if (digit1 >= 0 && digit2 >= 0) {
+            int byte = digit1 * 16 + digit2;
+            result[index++] = byte;
+            i += 2;
+            continue;
+          }
+        }
+        throw new FormatException("Invalid percent escape", text, i);
+      }
+    }
+    assert(index == result.length);
+    return result;
+  }
+
+  /**
+   * Returns a string created from the content of the data URI.
+   *
+   * If the content is Base64 encoded, it will be decoded to bytes and then
+   * decoded to a string using [encoding].
+   * If encoding is omitted, the value of a `charset` parameter is used
+   * if it is recongized by [Encoding.getByName], otherwise it defaults to
+   * the [ASCII] encoding, which is the default encoding for data URIs
+   * that do not specify an encoding.
+   *
+   * If the content is not Base64 encoded, it will first have percent-escapes
+   * converted to bytes and then the character codes and byte values are
+   * decoded using [encoding].
+   */
+  String contentAsString({Encoding encoding}) {
+    if (encoding == null) {
+      var charset = this.charset;  // Returns "US-ASCII" if not present.
+      encoding = Encoding.getByName(charset);
+      if (encoding == null) {
+        throw new UnsupportedError("Unknown charset: $charset");
+      }
+    }
+    String text = _text;
+    int start = _separatorIndices.last + 1;
+    if (isBase64) {
+      var converter = BASE64.decoder.fuse(encoding.decoder);
+      return converter.convert(text.substring(start));
+    }
+    return Uri._uriDecode(text, start: start, encoding: encoding);
+  }
+
+  /**
+   * A map representing the parameters of the media type.
+   *
+   * A data URI may contain parameters between the the MIME type and the
+   * data. This converts these parameters to a map from parameter name
+   * to parameter value.
+   * The map only contains parameters that actually occur in the URI.
+   * The `charset` parameter has a default value even if it doesn't occur
+   * in the URI, which is reflected by the [charset] getter. This means that
+   * [charset] may return a value even if `parameters["charset"]` is `null`.
+   *
+   * If the values contain non-ASCII values or percent escapes, they default
+   * to being decoded as UTF-8.
+   */
+  Map<String, String> get parameters {
+    var result = <String, String>{};
+    for (int i = 3; i < _separatorIndices.length; i += 2) {
+      var start = _separatorIndices[i - 2] + 1;
+      var equals = _separatorIndices[i - 1];
+      var end = _separatorIndices[i];
+      String key = Uri._uriDecode(_text, start: start, end: equals);
+      String value = Uri._uriDecode(_text, start: equals + 1, end: end);
+      result[key] = value;
+    }
+    return result;
+  }
+
+  static UriData _parse(String text, int start, Uri sourceUri) {
+    assert(start == 0 || start == 5);
+    assert((start == 5) == text.startsWith("data:"));
+
+    /// Character codes.
+    const int comma     = 0x2c;
+    const int slash     = 0x2f;
+    const int semicolon = 0x3b;
+    const int equals    = 0x3d;
+    List indices = [start - 1];
+    int slashIndex = -1;
+    var char;
+    int i = start;
+    for (; i < text.length; i++) {
+      char = text.codeUnitAt(i);
+      if (char == comma || char == semicolon) break;
+      if (char == slash) {
+        if (slashIndex < 0) {
+          slashIndex = i;
+          continue;
+        }
+        throw new FormatException("Invalid MIME type", text, i);
+      }
+    }
+    if (slashIndex < 0 && i > start) {
+      // An empty MIME type is allowed, but if non-empty it must contain
+      // exactly one slash.
+      throw new FormatException("Invalid MIME type", text, i);
+    }
+    while (char != comma) {
+      // parse parameters and/or "base64".

[floitsch, 2015/11/07 02:56:27]

Nit: "Parse"

[Lasse Reichstein Nielsen, 2015/11/09 10:27:38]

Done.

+      indices.add(i);
+      i++;
+      int equalsIndex = -1;
+      for (; i < text.length; i++) {
+        char = text.codeUnitAt(i);
+        if (char == equals) {
+          if (equalsIndex < 0) equalsIndex = i;
+        } else if (char == semicolon || char == comma) {
+          break;
+        }
+      }
+      if (equalsIndex >= 0) {
+        indices.add(equalsIndex);
+      } else {
+        // Have to be final "base64".
+        var lastSeparator = indices.last;
+        if (char != comma ||
+            i != lastSeparator + 7 /* "base64,".length */ ||
+            !text.startsWith("base64", lastSeparator + 1)) {
+          throw new FormatException("Expecting '='", text, i);
+        }
+        break;
+      }
+    }
+    indices.add(i);
+    return new UriData._(text, indices, sourceUri);
+  }
+
+  /**
+   * Like [Uri._uriEncode] but takes the input as bytes, not a string.
+   *
+   * Encodes into [buffer] instead of creating its own buffer.
+   */
+  static void _uriEncodeBytes(List<int> canonicalTable,
+                              List<int> bytes,
+                              StringSink buffer) {
+    // Encode the string into bytes then generate an ASCII only string
+    // by percent encoding selected bytes.
+    int byteOr = 0;
+    for (int i = 0; i < bytes.length; i++) {
+      int byte = bytes[i];
+      byteOr |= byte;
+      if (byte < 128 &&
+          ((canonicalTable[byte >> 4] & (1 << (byte & 0x0f))) != 0)) {
+        buffer.writeCharCode(byte);
+      } else {
+        buffer.writeCharCode(Uri._PERCENT);
+        buffer.writeCharCode(Uri._hexDigits.codeUnitAt(byte >> 4));
+        buffer.writeCharCode(Uri._hexDigits.codeUnitAt(byte & 0x0f));
+      }
+    }
+    if ((byteOr & ~0xFF) != 0) {
+      for (int i = 0; i < bytes.length; i++) {
+        var byte = bytes[i];
+        if (byte < 0 || byte > 255) {
+          throw new ArgumentError.value(byte, "non-byte value");
+        }
+      }
+    }
+  }
+
+  String toString() =>
+      (_separatorIndices[0] == _noScheme) ? "data:$_text" : _text;
+
+  // Table of the `token` characters of RFC 2045 in a URI.
+  //
+  // A token is any US-ASCII character except SPACE, control characters and
+  // `tspecial` characters. The `tspecial` category is:
+  // '(', ')', '<', '>', '@', ',', ';', ':', '\', '"', '/', '[, ']', '?', '='.
+  //
+  // In a data URI, we also need to escape '%' and '#' characters.
+  static const _tokenCharTable = const [
+                //             LSB             MSB
+                //              |               |
+      0x0000,   // 0x00 - 0x0f  00000000 00000000
+      0x0000,   // 0x10 - 0x1f  00000000 00000000
+                //               !  $ &'   *+ -.
+      0x6cd2,   // 0x20 - 0x2f  01001011 00110110
+                //              01234567 89
+      0x03ff,   // 0x30 - 0x3f  11111111 11000000
+                //               ABCDEFG HIJKLMNO
+      0xfffe,   // 0x40 - 0x4f  01111111 11111111
+                //              PQRSTUVW XYZ   ^_
+      0xc7ff,   // 0x50 - 0x5f  11111111 11100011
+                //              `abcdefg hijklmno
+      0xffff,   // 0x60 - 0x6f  11111111 11111111
+                //              pqrstuvw xyz{|}~
+      0x7fff];  // 0x70 - 0x7f  11111111 11111110
+
+  // All non-escape RFC-2396 uric characters.
+  //
+  //  uric        =  reserved | unreserved | escaped
+  //  reserved    =  ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" | "$" | ","
+  //  unreserved  =  alphanum | mark
+  //  mark        =  "-" | "_" | "." | "!" | "~" | "*" | "'" | "(" | ")"
+  //
+  // This is the same characters as in a URI query (which is URI pchar plus '?')
+  static const _uricTable = Uri._queryCharTable;
 }