Use the dart2js patch for dart:convert to get latest type fixes

pkg/dev_compiler/tool/input_sdk/patch/convert_patch.dart

Index: pkg/dev_compiler/tool/input_sdk/patch/convert_patch.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/patch/convert_patch.dart b/pkg/dev_compiler/tool/input_sdk/patch/convert_patch.dart
index 41130a9973689fdd0f4c497d5e99372d82dd1261..e88ac32b3ad3c482dfb165a0170045684b280384 100644
--- a/pkg/dev_compiler/tool/input_sdk/patch/convert_patch.dart
+++ b/pkg/dev_compiler/tool/input_sdk/patch/convert_patch.dart
@@ -123,7 +123,7 @@ _convertJsonToDartLazy(object) {
   return object;
 }
 
-class _JsonMap implements LinkedHashMap {
+class _JsonMap implements Map<String, dynamic> {
   // The original JavaScript object remains unchanged until
   // the map is eventually upgraded, in which case we null it
   // out to reclaim the memory used by it.
@@ -156,7 +156,7 @@ class _JsonMap implements LinkedHashMap {
   bool get isEmpty => length == 0;
   bool get isNotEmpty => length > 0;
 
-  Iterable get keys {
+  Iterable<String> get keys {
     if (_isUpgraded) return _upgradedMap.keys;
     return new _JsonMapKeyIterable(this);
   }
@@ -280,12 +280,12 @@ class _JsonMap implements LinkedHashMap {
     return JS('JSExtendableArray', '#', keys);
   }
 
-  Map _upgrade() {
+  Map<String, dynamic> _upgrade() {
     if (_isUpgraded) return _upgradedMap;
 
     // Copy all the (key, value) pairs to a freshly allocated
     // linked hash map thus preserving the ordering.
-    Map result = {};
+    Map result = <String, dynamic>{};

[vsm, 2017/05/24 00:02:18]

If we have Map here, the static type of result is Map<dynamic, dynamic>,
resulting in a runtime check below.

     List<String> keys = _computeKeys();
     for (int i = 0; i < keys.length; i++) {
       String key = keys[i];
@@ -331,7 +331,7 @@ class _JsonMap implements LinkedHashMap {
   static _newJavaScriptObject() => JS('=Object', 'Object.create(null)');
 }
 
-class _JsonMapKeyIterable extends ListIterable {
+class _JsonMapKeyIterable extends ListIterable<String> {
   final _JsonMap _parent;
 
   _JsonMapKeyIterable(this._parent);
@@ -347,7 +347,7 @@ class _JsonMapKeyIterable extends ListIterable {
   /// Although [ListIterable] defines its own iterator, we return the iterator
   /// of the underlying list [_keys] in order to propagate
   /// [ConcurrentModificationError]s.
-  Iterator get iterator {
+  Iterator<String> get iterator {
     return _parent._isUpgraded
         ? _parent.keys.iterator
         : _parent._computeKeys().iterator;
@@ -377,7 +377,7 @@ class _JsonDecoderSink extends _StringSinkConversionSink {
   final _Reviver _reviver;
   final Sink<Object> _sink;
 
-  _JsonDecoderSink(this._reviver, this._sink) : super(new StringBuffer());
+  _JsonDecoderSink(this._reviver, this._sink) : super(new StringBuffer(''));
 
   void close() {
     super.close();
@@ -393,9 +393,8 @@ class _JsonDecoderSink extends _StringSinkConversionSink {
 @patch
 class Utf8Decoder {
   @patch
-  Converter<List<int>, dynamic/*=T*/ > fuse/*<T>*/(
-      Converter<String, dynamic/*=T*/ > next) {
-    return super.fuse/*<T>*/(next);
+  Converter<List<int>, T> fuse<T>(Converter<String, T> next) {
+    return super.fuse(next);
   }
 
   // Currently not intercepting UTF8 decoding.