don't run dart gen on JS patch code

test/dart_codegen/expect/core/string.dart

Index: test/dart_codegen/expect/core/string.dart
diff --git a/test/dart_codegen/expect/core/string.dart b/test/dart_codegen/expect/core/string.dart
index 62925b967d407684cf7a547f0ace6c7cf056f710..31cf22585d95a4421a113805001c4b72ef98a6ee 100644
--- a/test/dart_codegen/expect/core/string.dart
+++ b/test/dart_codegen/expect/core/string.dart
@@ -1,42 +1,11 @@
 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;
@@ -116,9 +85,9 @@ class RuneIterator implements BidirectionalIterator<int> {
       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$RT.cast(__x9, dynamic, int, "CastGeneral",
+      """line 665, column 24 of dart:core/string.dart: """, __x9 is int,
+      true))((_position != _nextPosition) ? _position : null);
   void set rawIndex(int rawIndex) {
     RangeError.checkValidIndex(rawIndex, string, "rawIndex");
     reset(rawIndex);
@@ -131,7 +100,7 @@ class RuneIterator implements BidirectionalIterator<int> {
     _currentCodePoint = null;
   }
   int get current => DDC$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 {