don't run dart gen on JS patch code

test/dart_codegen/expect/core/string.dart

 part of dart.core;
 
 abstract class String implements Comparable<String>, Pattern {
-  factory String.fromCharCodes(Iterable<int> charCodes,
-      [int start = 0, int end]) {
-    if (charCodes is! JSArray) {
-      return ((__x36) => DDC$RT.cast(__x36, dynamic, String, "CastGeneral",
-          """line 118, column 15 of dart:core/string.dart: """, __x36 is String,
-          true))(_stringFromIterable(charCodes, start, end));
-    }
-    List list = DDC$RT.cast(charCodes, DDC$RT.type((Iterable<int> _) {}),
-        DDC$RT.type((List<dynamic> _) {}), "CastGeneral",
-        """line 121, column 18 of dart:core/string.dart: """,
-        charCodes is List<dynamic>, true);
-    int len = list.length;
-    if (start < 0 || start > len) {
-      throw new RangeError.range(start, 0, len);
-    }
-    if (end == null) {
-      end = len;
-    } else if (end < start || end > len) {
-      throw new RangeError.range(end, start, len);
-    }
-    if (start > 0 || end < len) {
-      list = list.sublist(start, end);
-    }
-    return ((__x37) => DDC$RT.cast(__x37, dynamic, String, "CastGeneral",
-        """line 135, column 13 of dart:core/string.dart: """, __x37 is String,
-        true))(Primitives.stringFromCharCodes(list));
-  }
-  factory String.fromCharCode(int charCode) {
-    return ((__x38) => DDC$RT.cast(__x38, dynamic, String, "CastGeneral",
-        """line 149, column 13 of dart:core/string.dart: """, __x38 is String,
-        true))(Primitives.stringFromCharCode(charCode));
-  }
-  factory String.fromEnvironment(String name, {String defaultValue}) {
-    throw new UnsupportedError(
-        'String.fromEnvironment can only be used as a const constructor');
-  }
+  external factory String.fromCharCodes(Iterable<int> charCodes,
+      [int start = 0, int end]);
+  external factory String.fromCharCode(int charCode);
+  external const factory String.fromEnvironment(String name,
+      {String defaultValue});
   String operator [](int index);
   int codeUnitAt(int index);
   int get length;
   int get hashCode;
   bool operator ==(Object other);
   bool endsWith(String other);
   bool startsWith(Pattern pattern, [int index = 0]);
   int indexOf(Pattern pattern, [int start]);
   int lastIndexOf(Pattern pattern, [int start]);
   bool get isEmpty;
@@ skipping 59 matching lines @@
     _checkSplitSurrogate(index);
   }
   void _checkSplitSurrogate(int index) {
     if (index > 0 &&
         index < string.length &&
         _isLeadSurrogate(string.codeUnitAt(index - 1)) &&
         _isTrailSurrogate(string.codeUnitAt(index))) {
       throw new ArgumentError('Index inside surrogate pair: $index');
     }
   }
-  int get rawIndex => ((__x39) => DDC$RT.cast(__x39, dynamic, int,
-      "CastGeneral", """line 690, column 24 of dart:core/string.dart: """,
-      __x39 is int, true))((_position != _nextPosition) ? _position : null);
+  int get rawIndex => ((__x9) => DDC$RT.cast(__x9, dynamic, int, "CastGeneral",
+      """line 665, column 24 of dart:core/string.dart: """, __x9 is int,
+      true))((_position != _nextPosition) ? _position : null);
   void set rawIndex(int rawIndex) {
     RangeError.checkValidIndex(rawIndex, string, "rawIndex");
     reset(rawIndex);
     moveNext();
   }
   void reset([int rawIndex = 0]) {
     RangeError.checkValueInInterval(rawIndex, 0, string.length, "rawIndex");
     _checkSplitSurrogate(rawIndex);
     _position = _nextPosition = rawIndex;
     _currentCodePoint = null;
   }
   int get current => DDC$RT.cast(_currentCodePoint, num, int, "CastGeneral",
-      """line 727, column 23 of dart:core/string.dart: """,
+      """line 702, column 23 of dart:core/string.dart: """,
       _currentCodePoint is int, true);
   int get currentSize => _nextPosition - _position;
   String get currentAsString {
     if (_position == _nextPosition) return null;
     if (_position + 1 == _nextPosition) return string[_position];
     return string.substring(_position, _nextPosition);
   }
   bool moveNext() {
     _position = _nextPosition;
     if (_position == string.length) {
@@ skipping 28 matching lines @@
         _position = position - 1;
         _currentCodePoint = _combineSurrogatePair(prevCodeUnit, codeUnit);
         return true;
       }
     }
     _position = position;
     _currentCodePoint = codeUnit;
     return true;
   }
 }