Move json, uri, utf and crypto libraries into the VM.

sdk/lib/json/json_base.dart

Index: sdk/lib/json/json_base.dart
===================================================================
--- sdk/lib/json/json_base.dart	(revision 0)
+++ sdk/lib/json/json_base.dart	(revision 0)
@@ -0,0 +1,802 @@
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// JSON parsing and serialization.
+
+/**
+ * Error thrown by JSON serialization if an object cannot be serialized.
+ *
+ * The [unsupportedObject] field holds that object that failed to be serialized.
+ *
+ * If an object isn't directly serializable, the serializer calls the 'toJson'
+ * method on the object. If that call fails, the error will be stored in the
+ * [cause] field. If the call returns an object that isn't directly
+ * serializable, the [cause] will be null.
+ */
+class JsonUnsupportedObjectError implements Error {
+  /** The object that could not be serialized. */
+  final unsupportedObject;
+  /** The exception thrown by object's [:toJson:] method, if any. */
+  final cause;
+  JsonUnsupportedObjectError(this.unsupportedObject) : cause = null;
+  JsonUnsupportedObjectError.withCause(this.unsupportedObject, this.cause);
+
+  String toString() {
+    if (cause != null) {
+      return "Calling toJson method on object failed.";
+    } else {
+      return "Object toJson method returns non-serializable value.";
+    }
+  }
+}
+
+
+/**
+ * Parses [json] and build the corresponding parsed JSON value.
+ *
+ * Parsed JSON values are of the types [num], [String], [bool], [Null],
+ * [List]s of parsed JSON values or [Map]s from [String] to parsed
+ * JSON values.
+ *
+ * The optional [revivier] function, if provided, is called once for each
+ * object or list property parsed. The arguments are the property name
+ * ([String]) or list index ([int]), and the value is the parsed value.
+ * The return value of the revivier will be used as the value of that property
+ * instead the parsed value.
+ *
+ * Throws [FormatException] if the input is not valid JSON text.
+ */
+parse(String json, [reviver(var key, var value)]) {
+  BuildJsonListener listener;
+  if (reviver == null) {
+    listener = new BuildJsonListener();
+  } else {
+    listener = new ReviverJsonListener(reviver);
+  }
+  new JsonParser(json, listener).parse();
+  return listener.result;
+}
+
+/**
+ * Serializes [object] into a JSON string.
+ *
+ * Directly serializable types are [num], [String], [bool], [Null], [List]
+ * and [Map].
+ * For [List], the elements must all be serializable.
+ * For [Map], the keys must be [String] and the values must be serializable.
+ * If a value is any other type is attempted serialized, a "toJson()" method
+ * is invoked on the object and the result, which must be a directly
+ * serializable type, is serialized instead of the original value.
+ * If the object does not support this method, throws, or returns a
+ * value that is not directly serializable, a [JsonUnsupportedObjectError]
+ * exception is thrown. If the call throws (including the case where there
+ * is no nullary "toJson" method, the error is caught and stored in the
+ * [JsonUnsupportedObjectError]'s [:cause:] field.
+ *Json
+ * Objects should not change during serialization.
+ * If an object is serialized more than once, [stringify] is allowed to cache
+ * the JSON text for it. I.e., if an object changes after it is first
+ * serialized, the new values may or may not be reflected in the result.
+ */
+String stringify(Object object) {
+  return _JsonStringifier.stringify(object);
+}
+
+/**
+ * Serializes [object] into [output] stream.
+ *
+ * Performs the same operations as [stringify] but outputs the resulting
+ * string to an existing [StringBuffer] instead of creating a new [String].
+ *
+ * If serialization fails by throwing, some data might have been added to
+ * [output], but it won't contain valid JSON text.
+ */
+void printOn(Object object, StringBuffer output) {
+  return _JsonStringifier.printOn(object, output);
+}
+
+//// Implementation ///////////////////////////////////////////////////////////
+
+// Simple API for JSON parsing.
+
+abstract class JsonListener {
+  void handleString(String value) {}
+  void handleNumber(num value) {}
+  void handleBool(bool value) {}
+  void handleNull() {}
+  void beginObject() {}
+  void propertyName() {}
+  void propertyValue() {}
+  void endObject() {}
+  void beginArray() {}
+  void arrayElement() {}
+  void endArray() {}
+  /** Called on failure to parse [source]. */
+  void fail(String source, int position, String message) {}
+}
+
+/**
+ * A [JsonListener] that builds data objects from the parser events.
+ *
+ * This is a simple stack-based object builder. It keeps the most recently
+ * seen value in a variable, and uses it depending on the following event.
+ */
+class BuildJsonListener extends JsonListener {
+  /**
+   * Stack used to handle nested containers.
+   *
+   * The current container is pushed on the stack when a new one is
+   * started. If the container is a [Map], there is also a current [key]
+   * which is also stored on the stack.
+   */
+  List stack = [];
+  /** The current [Map] or [List] being built. */
+  var currentContainer;
+  /** The most recently read property key. */
+  String key;
+  /** The most recently read value. */
+  var value;
+
+  /** Pushes the currently active container (and key, if a [Map]). */
+  void pushContainer() {
+    if (currentContainer is Map) stack.add(key);
+    stack.add(currentContainer);
+  }
+
+  /** Pops the top container from the [stack], including a key if applicable. */
+  void popContainer() {
+    value = currentContainer;
+    currentContainer = stack.removeLast();
+    if (currentContainer is Map) key = stack.removeLast();
+  }
+
+  void handleString(String value) { this.value = value; }
+  void handleNumber(num value) { this.value = value; }
+  void handleBool(bool value) { this.value = value; }
+  void handleNull() { this.value = value; }
+
+  void beginObject() {
+    pushContainer();
+    currentContainer = {};
+  }
+
+  void propertyName() {
+    key = value;
+    value = null;
+  }
+
+  void propertyValue() {
+    Map map = currentContainer;
+    map[key] = value;
+    key = value = null;
+  }
+
+  void endObject() {
+    popContainer();
+  }
+
+  void beginArray() {
+    pushContainer();
+    currentContainer = [];
+  }
+
+  void arrayElement() {
+    List list = currentContainer;
+    currentContainer.add(value);
+    value = null;
+  }
+
+  void endArray() {
+    popContainer();
+  }
+
+  /** Read out the final result of parsing a JSON string. */
+  get result {
+    assert(currentContainer == null);
+    return value;
+  }
+}
+
+typedef _Reviver(var key, var value);
+
+class ReviverJsonListener extends BuildJsonListener {
+  final _Reviver reviver;
+  ReviverJsonListener(reviver(key, value)) : this.reviver = reviver;
+
+  void arrayElement() {
+    List list = currentContainer;
+    value = reviver(list.length, value);
+    super.arrayElement();
+  }
+
+  void propertyValue() {
+    value = reviver(key, value);
+    super.propertyValue();
+  }
+
+  get result {
+    return reviver("", value);
+  }
+}
+
+class JsonParser {
+  // A simple non-recursive state-based parser for JSON.
+  //
+  // Literal values accepted in states ARRAY_EMPTY, ARRAY_COMMA, OBJECT_COLON
+  // and strings also in OBJECT_EMPTY, OBJECT_COMMA.
+  //               VALUE  STRING  :  ,  }  ]        Transitions to
+  // EMPTY            X      X                   -> END
+  // ARRAY_EMPTY      X      X             @     -> ARRAY_VALUE / pop
+  // ARRAY_VALUE                     @     @     -> ARRAY_COMMA / pop
+  // ARRAY_COMMA      X      X                   -> ARRAY_VALUE
+  // OBJECT_EMPTY            X          @        -> OBJECT_KEY / pop
+  // OBJECT_KEY                   @              -> OBJECT_COLON
+  // OBJECT_COLON     X      X                   -> OBJECT_VALUE
+  // OBJECT_VALUE                    @  @        -> OBJECT_COMMA / pop
+  // OBJECT_COMMA            X                   -> OBJECT_KEY
+  // END
+  // Starting a new array or object will push the current state. The "pop"
+  // above means restoring this state and then marking it as an ended value.
+  // X means generic handling, @ means special handling for just that
+  // state - that is, values are handled generically, only punctuation
+  // cares about the current state.
+  // Values for states are chosen so bits 0 and 1 tell whether
+  // a string/value is allowed, and setting bits 0 through 2 after a value
+  // gets to the next state (not empty, doesn't allow a value).
+
+  // State building-block constants.
+  static const int INSIDE_ARRAY = 1;
+  static const int INSIDE_OBJECT = 2;
+  static const int AFTER_COLON = 3;  // Always inside object.
+
+  static const int ALLOW_STRING_MASK = 8;  // Allowed if zero.
+  static const int ALLOW_VALUE_MASK = 4;  // Allowed if zero.
+  static const int ALLOW_VALUE = 0;
+  static const int STRING_ONLY = 4;
+  static const int NO_VALUES = 12;
+
+  // Objects and arrays are "empty" until their first property/element.
+  static const int EMPTY = 0;
+  static const int NON_EMPTY = 16;
+  static const int EMPTY_MASK = 16;  // Empty if zero.
+
+
+  static const int VALUE_READ_BITS = NO_VALUES | NON_EMPTY;
+
+  // Actual states.
+  static const int STATE_INITIAL      = EMPTY | ALLOW_VALUE;
+  static const int STATE_END          = NON_EMPTY | NO_VALUES;
+
+  static const int STATE_ARRAY_EMPTY  = INSIDE_ARRAY | EMPTY | ALLOW_VALUE;
+  static const int STATE_ARRAY_VALUE  = INSIDE_ARRAY | NON_EMPTY | NO_VALUES;
+  static const int STATE_ARRAY_COMMA  = INSIDE_ARRAY | NON_EMPTY | ALLOW_VALUE;
+
+  static const int STATE_OBJECT_EMPTY = INSIDE_OBJECT | EMPTY | STRING_ONLY;
+  static const int STATE_OBJECT_KEY   = INSIDE_OBJECT | NON_EMPTY | NO_VALUES;
+  static const int STATE_OBJECT_COLON = AFTER_COLON | NON_EMPTY | ALLOW_VALUE;
+  static const int STATE_OBJECT_VALUE = AFTER_COLON | NON_EMPTY | NO_VALUES;
+  static const int STATE_OBJECT_COMMA = INSIDE_OBJECT | NON_EMPTY | STRING_ONLY;
+
+  // Character code constants.
+  static const int BACKSPACE       = 0x08;
+  static const int TAB             = 0x09;
+  static const int NEWLINE         = 0x0a;
+  static const int CARRIAGE_RETURN = 0x0d;
+  static const int FORM_FEED       = 0x0c;
+  static const int SPACE           = 0x20;
+  static const int QUOTE           = 0x22;
+  static const int PLUS            = 0x2b;
+  static const int COMMA           = 0x2c;
+  static const int MINUS           = 0x2d;
+  static const int DECIMALPOINT    = 0x2e;
+  static const int SLASH           = 0x2f;
+  static const int CHAR_0          = 0x30;
+  static const int CHAR_9          = 0x39;
+  static const int COLON           = 0x3a;
+  static const int CHAR_E          = 0x45;
+  static const int LBRACKET        = 0x5b;
+  static const int BACKSLASH       = 0x5c;
+  static const int RBRACKET        = 0x5d;
+  static const int CHAR_a          = 0x61;
+  static const int CHAR_b          = 0x62;
+  static const int CHAR_e          = 0x65;
+  static const int CHAR_f          = 0x66;
+  static const int CHAR_l          = 0x6c;
+  static const int CHAR_n          = 0x6e;
+  static const int CHAR_r          = 0x72;
+  static const int CHAR_s          = 0x73;
+  static const int CHAR_t          = 0x74;
+  static const int CHAR_u          = 0x75;
+  static const int LBRACE          = 0x7b;
+  static const int RBRACE          = 0x7d;
+
+  final String source;
+  final JsonListener listener;
+  JsonParser(this.source, this.listener);
+
+  /** Parses [source], or throws if it fails. */
+  void parse() {
+    final List<int> states = <int>[];
+    int state = STATE_INITIAL;
+    int position = 0;
+    int length = source.length;
+    while (position < length) {
+      int char = source.charCodeAt(position);
+      switch (char) {
+        case SPACE:
+        case CARRIAGE_RETURN:
+        case NEWLINE:
+        case TAB:
+          position++;
+          break;
+        case QUOTE:
+          if ((state & ALLOW_STRING_MASK) != 0) fail(position);
+          position = parseString(position + 1);
+          state |= VALUE_READ_BITS;
+          break;
+        case LBRACKET:
+          if ((state & ALLOW_VALUE_MASK) != 0) fail(position);
+          listener.beginArray();
+          states.add(state);
+          state = STATE_ARRAY_EMPTY;
+          position++;
+          break;
+        case LBRACE:
+          if ((state & ALLOW_VALUE_MASK) != 0) fail(position);
+          listener.beginObject();
+          states.add(state);
+          state = STATE_OBJECT_EMPTY;
+          position++;
+          break;
+        case CHAR_n:
+          if ((state & ALLOW_VALUE_MASK) != 0) fail(position);
+          position = parseNull(position);
+          state |= VALUE_READ_BITS;
+          break;
+        case CHAR_f:
+          if ((state & ALLOW_VALUE_MASK) != 0) fail(position);
+          position = parseFalse(position);
+          state |= VALUE_READ_BITS;
+          break;
+        case CHAR_t:
+          if ((state & ALLOW_VALUE_MASK) != 0) fail(position);
+          position = parseTrue(position);
+          state |= VALUE_READ_BITS;
+          break;
+        case COLON:
+          if (state != STATE_OBJECT_KEY) fail(position);
+          listener.propertyName();
+          state = STATE_OBJECT_COLON;
+          position++;
+          break;
+        case COMMA:
+          if (state == STATE_OBJECT_VALUE) {
+            listener.propertyValue();
+            state = STATE_OBJECT_COMMA;
+            position++;
+          } else if (state == STATE_ARRAY_VALUE) {
+            listener.arrayElement();
+            state = STATE_ARRAY_COMMA;
+            position++;
+          } else {
+            fail(position);
+          }
+          break;
+        case RBRACKET:
+          if (state == STATE_ARRAY_EMPTY) {
+            listener.endArray();
+          } else if (state == STATE_ARRAY_VALUE) {
+            listener.arrayElement();
+            listener.endArray();
+          } else {
+            fail(position);
+          }
+          state = states.removeLast() | VALUE_READ_BITS;
+          position++;
+          break;
+        case RBRACE:
+          if (state == STATE_OBJECT_EMPTY) {
+            listener.endObject();
+          } else if (state == STATE_OBJECT_VALUE) {
+            listener.propertyValue();
+            listener.endObject();
+          } else {
+            fail(position);
+          }
+          state = states.removeLast() | VALUE_READ_BITS;
+          position++;
+          break;
+        default:
+          if ((state & ALLOW_VALUE_MASK) != 0) fail(position);
+          position = parseNumber(char, position);
+          state |= VALUE_READ_BITS;
+          break;
+      }
+    }
+    if (state != STATE_END) fail(position);
+  }
+
+  /**
+   * Parses a "true" literal starting at [position].
+   *
+   * [:source[position]:] must be "t".
+   */
+  int parseTrue(int position) {
+    assert(source.charCodeAt(position) == CHAR_t);
+    if (source.length < position + 4) fail(position, "Unexpected identifier");
+    if (source.charCodeAt(position + 1) != CHAR_r ||
+        source.charCodeAt(position + 2) != CHAR_u ||
+        source.charCodeAt(position + 3) != CHAR_e) {
+      fail(position);
+    }
+    listener.handleBool(true);
+    return position + 4;
+  }
+
+  /**
+   * Parses a "false" literal starting at [position].
+   *
+   * [:source[position]:] must be "f".
+   */
+  int parseFalse(int position) {
+    assert(source.charCodeAt(position) == CHAR_f);
+    if (source.length < position + 5) fail(position, "Unexpected identifier");
+    if (source.charCodeAt(position + 1) != CHAR_a ||
+        source.charCodeAt(position + 2) != CHAR_l ||
+        source.charCodeAt(position + 3) != CHAR_s ||
+        source.charCodeAt(position + 4) != CHAR_e) {
+      fail(position);
+    }
+    listener.handleBool(false);
+    return position + 5;
+  }
+
+  /** Parses a "null" literal starting at [position].
+   *
+   * [:source[position]:] must be "n".
+   */
+  int parseNull(int position) {
+    assert(source.charCodeAt(position) == CHAR_n);
+    if (source.length < position + 4) fail(position, "Unexpected identifier");
+    if (source.charCodeAt(position + 1) != CHAR_u ||
+        source.charCodeAt(position + 2) != CHAR_l ||
+        source.charCodeAt(position + 3) != CHAR_l) {
+      fail(position);
+    }
+    listener.handleNull();
+    return position + 4;
+  }
+
+  int parseString(int position) {
+    // Format: '"'([^\x00-\x1f\\\"]|'\\'[bfnrt/\\"])*'"'
+    // Initial position is right after first '"'.
+    int start = position;
+    int char;
+    do {
+      if (position == source.length) {
+        fail(start - 1, "Unterminated string");
+      }
+      char = source.charCodeAt(position);
+      if (char == QUOTE) {
+        listener.handleString(source.substring(start, position));
+        return position + 1;
+      }
+      if (char < SPACE) {
+        fail(position, "Control character in string");
+      }
+      position++;
+    } while (char != BACKSLASH);
+    // Backslash escape detected. Collect character codes for rest of string.
+    int firstEscape = position - 1;
+    List<int> chars = <int>[];
+    while (true) {
+      if (position == source.length) {
+        fail(start - 1, "Unterminated string");
+      }
+      char = source.charCodeAt(position);
+      switch (char) {
+        case CHAR_b: char = BACKSPACE; break;
+        case CHAR_f: char = FORM_FEED; break;
+        case CHAR_n: char = NEWLINE; break;
+        case CHAR_r: char = CARRIAGE_RETURN; break;
+        case CHAR_t: char = TAB; break;
+        case SLASH:
+        case BACKSLASH:
+        case QUOTE:
+          break;
+        case CHAR_u:
+          int hexStart = position - 1;
+          int value = 0;
+          for (int i = 0; i < 4; i++) {
+            position++;
+            if (position == source.length) {
+              fail(start - 1, "Unterminated string");
+            }
+            char = source.charCodeAt(position);
+            char -= 0x30;
+            if (char < 0) fail(hexStart, "Invalid unicode escape");
+            if (char < 10) {
+              value = value * 16 + char;
+            } else {
+              char = (char | 0x20) - 0x31;
+              if (char < 0 || char > 5) {
+                fail(hexStart, "Invalid unicode escape");
+              }
+              value = value * 16 + char + 10;
+            }
+          }
+          char = value;
+          break;
+        default:
+          if (char < SPACE) fail(position, "Control character in string");
+          fail(position, "Unrecognized string escape");
+      }
+      do {
+        chars.add(char);
+        position++;
+        if (position == source.length) fail(start - 1, "Unterminated string");
+        char = source.charCodeAt(position);
+        if (char == QUOTE) {
+          String result = new String.fromCharCodes(chars);
+          if (start < firstEscape) {
+            result = "${source.substring(start, firstEscape)}$result";
+          }
+          listener.handleString(result);
+          return position + 1;
+        }
+        if (char < SPACE) {
+          fail(position, "Control character in string");
+        }
+      } while (char != BACKSLASH);
+      position++;
+    }
+  }
+
+  int parseNumber(int char, int position) {
+    // Format:
+    //  '-'?('0'|[1-9][0-9]*)('.'[0-9]+)?([eE][+-]?[0-9]+)?
+    int start = position;
+    int length = source.length;
+    bool isDouble = false;
+    if (char == MINUS) {
+      position++;
+      if (position == length) fail(position, "Missing expected digit");
+      char = source.charCodeAt(position);
+    }
+    if (char < CHAR_0 || char > CHAR_9) {
+      fail(position, "Missing expected digit");
+    }
+    int handleLiteral(position) {
+      String literal = source.substring(start, position);
+      // This correctly creates -0 for doubles.
+      num value = (isDouble ? double.parse(literal) : int.parse(literal));
+      listener.handleNumber(value);
+      return position;
+    }
+    if (char == CHAR_0) {
+      position++;
+      if (position == length) return handleLiteral(position);
+      char = source.charCodeAt(position);
+      if (CHAR_0 <= char && char <= CHAR_9) {
+        fail(position);
+      }
+    } else {
+      do {
+        position++;
+        if (position == length) return handleLiteral(position);
+        char = source.charCodeAt(position);
+      } while (CHAR_0 <= char && char <= CHAR_9);
+    }
+    if (char == DECIMALPOINT) {
+      isDouble = true;
+      position++;
+      if (position == length) fail(position, "Missing expected digit");
+      char = source.charCodeAt(position);
+      if (char < CHAR_0 || char > CHAR_9) fail(position);
+      do {
+        position++;
+        if (position == length) return handleLiteral(position);
+        char = source.charCodeAt(position);
+      } while (CHAR_0 <= char && char <= CHAR_9);
+    }
+    if (char == CHAR_e || char == CHAR_E) {
+      isDouble = true;
+      position++;
+      if (position == length) fail(position, "Missing expected digit");
+      char = source.charCodeAt(position);
+      if (char == PLUS || char == MINUS) {
+        position++;
+        if (position == length) fail(position, "Missing expected digit");
+        char = source.charCodeAt(position);
+      }
+      if (char < CHAR_0 || char > CHAR_9) {
+        fail(position, "Missing expected digit");
+      }
+      do {
+        position++;
+        if (position == length) return handleLiteral(position);
+        char = source.charCodeAt(position);
+      } while (CHAR_0 <= char && char <= CHAR_9);
+    }
+    return handleLiteral(position);
+  }
+
+  void fail(int position, [String message]) {
+    if (message == null) message = "Unexpected character";
+    listener.fail(source, position, message);
+    // If the listener didn't throw, do it here.
+    String slice;
+    int sliceEnd = position + 20;
+    if (sliceEnd > source.length) {
+      slice = "'${source.substring(position)}'";
+    } else {
+      slice = "'${source.substring(position, sliceEnd)}...'";
+    }
+    throw new FormatException("Unexpected character at $position: $slice");
+  }
+}
+
+
+class _JsonStringifier {
+  StringBuffer sb;
+  List<Object> seen;  // TODO: that should be identity set.
+
+  _JsonStringifier(this.sb) : seen = [];
+
+  static String stringify(final object) {
+    StringBuffer output = new StringBuffer();
+    _JsonStringifier stringifier = new _JsonStringifier(output);
+    stringifier.stringifyValue(object);
+    return output.toString();
+  }
+
+  static void printOn(final object, StringBuffer output) {
+    _JsonStringifier stringifier = new _JsonStringifier(output);
+    stringifier.stringifyValue(object);
+  }
+
+  static String numberToString(num x) {
+    return x.toString();
+  }
+
+  // ('0' + x) or ('a' + x - 10)
+  static int hexDigit(int x) => x < 10 ? 48 + x : 87 + x;
+
+  static void escape(StringBuffer sb, String s) {
+    final int length = s.length;
+    bool needsEscape = false;
+    final charCodes = new List<int>();
+    for (int i = 0; i < length; i++) {
+      int charCode = s.charCodeAt(i);
+      if (charCode < 32) {
+        needsEscape = true;
+        charCodes.add(JsonParser.BACKSLASH);
+        switch (charCode) {
+        case JsonParser.BACKSPACE:
+          charCodes.add(JsonParser.CHAR_b);
+          break;
+        case JsonParser.TAB:
+          charCodes.add(JsonParser.CHAR_t);
+          break;
+        case JsonParser.NEWLINE:
+          charCodes.add(JsonParser.CHAR_n);
+          break;
+        case JsonParser.FORM_FEED:
+          charCodes.add(JsonParser.CHAR_f);
+          break;
+        case JsonParser.CARRIAGE_RETURN:
+          charCodes.add(JsonParser.CHAR_r);
+          break;
+        default:
+          charCodes.add(JsonParser.CHAR_u);
+          charCodes.add(hexDigit((charCode >> 12) & 0xf));
+          charCodes.add(hexDigit((charCode >> 8) & 0xf));
+          charCodes.add(hexDigit((charCode >> 4) & 0xf));
+          charCodes.add(hexDigit(charCode & 0xf));
+          break;
+        }
+      } else if (charCode == JsonParser.QUOTE ||
+          charCode == JsonParser.BACKSLASH) {
+        needsEscape = true;
+        charCodes.add(JsonParser.BACKSLASH);
+        charCodes.add(charCode);
+      } else {
+        charCodes.add(charCode);
+      }
+    }
+    sb.add(needsEscape ? new String.fromCharCodes(charCodes) : s);
+  }
+
+  void checkCycle(final object) {
+    // TODO: use Iterables.
+    for (int i = 0; i < seen.length; i++) {
+      if (identical(seen[i], object)) {
+        throw 'Cyclic structure';
+      }
+    }
+    seen.add(object);
+  }
+
+  void stringifyValue(final object) {
+    // Tries stringifying object directly. If it's not a simple value, List or
+    // Map, call toJson() to get a custom representation and try serializing
+    // that.
+    if (!stringifyJsonValue(object)) {
+      checkCycle(object);
+      try {
+        var customJson = object.toJson();
+        if (!stringifyJsonValue(customJson)) {
+          throw new JsonUnsupportedObjectError(object);
+        }
+        seen.removeLast();
+      } catch (e) {
+        throw new JsonUnsupportedObjectError.withCause(object, e);
+      }
+    }
+  }
+
+  /**
+   * Serializes a [num], [String], [bool], [Null], [List] or [Map] value.
+   *
+   * Returns true if the value is one of these types, and false if not.
+   * If a value is both a [List] and a [Map], it's serialized as a [List].
+   */
+  bool stringifyJsonValue(final object) {
+    if (object is num) {
+      // TODO: use writeOn.
+      sb.add(numberToString(object));
+      return true;
+    } else if (identical(object, true)) {
+      sb.add('true');
+      return true;
+    } else if (identical(object, false)) {
+      sb.add('false');
+       return true;
+    } else if (object == null) {
+      sb.add('null');
+      return true;
+    } else if (object is String) {
+      sb.add('"');
+      escape(sb, object);
+      sb.add('"');
+      return true;
+    } else if (object is List) {
+      checkCycle(object);
+      List a = object;
+      sb.add('[');
+      if (a.length > 0) {
+        stringifyValue(a[0]);
+        // TODO: switch to Iterables.
+        for (int i = 1; i < a.length; i++) {
+          sb.add(',');
+          stringifyValue(a[i]);
+        }
+      }
+      sb.add(']');
+      seen.removeLast();
+      return true;
+    } else if (object is Map) {
+      checkCycle(object);
+      Map<String, Object> m = object;
+      sb.add('{');
+      bool first = true;
+      m.forEach((String key, Object value) {
+        if (!first) {
+          sb.add(',"');
+        } else {
+          sb.add('"');
+        }
+        escape(sb, key);
+        sb.add('":');
+        stringifyValue(value);
+        first = false;
+      });
+      sb.add('}');
+      seen.removeLast();
+      return true;
+    } else {
+      return false;
+    }
+  }
+}