Replace old utf8 decoder with new one.

sdk/lib/codec/encoding.dart

Index: sdk/lib/codec/encoding.dart
diff --git a/sdk/lib/codec/encoding.dart b/sdk/lib/codec/encoding.dart
index 50b724be6d398d50ead3e91740080f43c604f47c..40e0f2b4d83ae56b3d58b217c06016e1a13485ad 100644
--- a/sdk/lib/codec/encoding.dart
+++ b/sdk/lib/codec/encoding.dart
@@ -20,11 +20,41 @@ const UTF8 = const Utf8Codec();
  * A [Utf8Codec] encodes strings to utf-8 code units (bytes) and decodes
  * UTF-8 code units to strings.
  */
-// TODO(floitsch): Needs a way to specify if decoding should throw or use
-// the replacement character.
 class Utf8Codec extends Encoding {
-  const Utf8Codec();
+  final bool _allowMalformed;
+
+  /**
+   * Instantiates a new [Utf8Codec].
+   *
+   * The optional [allowMalformed] argument defines how [decoder] (and [decode])
+   * deal with invalid or unterminated character sequences.
+   *
+   * If it is `true` (and not overriden at the method invocation) [decode] and
+   * the [decoder] replace invalid (or unterminated) octet
+   * sequences with the Unicode Replacement character `U+FFFD` (�). Otherwise
+   * they throw a [FormatException].
+   */
+  const Utf8Codec({ bool allowMalformed: false })
+      : _allowMalformed = allowMalformed;
+
+  /**
+   * Decodes the UTF-8 [codeUnits] (a list of unsigned 8-bit integers) to the
+   * corresponding string.
+   *
+   * If [allowMalformed] is `true` the decoder replaces invalid (or
+   * unterminated) character sequences with the Unicode Replacement character
+   * `U+FFFD` (�). Otherwise it throws a [FormatException].
+   *
+   * If [allowMalformed] is not given, it defaults to the `allowMalformed` that
+   * was used to instantiate `this`.
+   */
+  String decode(List<int> codeUnits, { bool allowMalformed }) {
+    if (allowMalformed == null) allowMalformed = _allowMalformed;
+    return new Utf8Decoder(allowMalformed: allowMalformed).convert(codeUnits);
+  }
 
   Converter<String, List<int>> get encoder => new Utf8Encoder();
-  Converter<List<int>, String> get decoder => new Utf8Decoder();
+  Converter<List<int>, String> get decoder {
+    return new Utf8Decoder(allowMalformed: _allowMalformed);
+  }
 }