don't run dart gen on JS patch code

test/dart_codegen/expect/core/exceptions.dart

 part of dart.core;
 
 abstract class Exception {
   factory Exception([var message]) => new _ExceptionImplementation(message);
 }
 class _ExceptionImplementation implements Exception {
   final message;
   _ExceptionImplementation([this.message]);
   String toString() {
     if (message == null) return "Exception";
@@ skipping 14 matching lines @@
     if (source is! String) {
       if (offset != -1) {
         report += " (at offset $offset)";
       }
       return report;
     }
     if (offset != -1 && (offset < 0 || offset > source.length)) {
       offset = -1;
     }
     if (offset == -1) {
-      String source = ((__x17) => DDC$RT.cast(__x17, dynamic, String,
+      String source = ((__x3) => DDC$RT.cast(__x3, dynamic, String,
           "CastGeneral",
           """line 113, column 23 of dart:core/exceptions.dart: """,
-          __x17 is String, true))(this.source);
+          __x3 is String, true))(this.source);
       if (source.length > 78) {
         source = source.substring(0, 75) + "...";
       }
       return "$report\n$source";
     }
     int lineNum = 1;
     int lineStart = 0;
     bool lastWasCR;
     for (int i = 0; i < offset; i++) {
-      int char = ((__x18) => DDC$RT.cast(__x18, dynamic, int, "CastGeneral",
-          """line 123, column 18 of dart:core/exceptions.dart: """,
-          __x18 is int, true))(source.codeUnitAt(i));
+      int char = ((__x4) => DDC$RT.cast(__x4, dynamic, int, "CastGeneral",
+          """line 123, column 18 of dart:core/exceptions.dart: """, __x4 is int,
+          true))(source.codeUnitAt(i));
       if (char == 0x0a) {
         if (lineStart != i || !lastWasCR) {
           lineNum++;
         }
         lineStart = i + 1;
         lastWasCR = false;
       } else if (char == 0x0d) {
         lineNum++;
         lineStart = i + 1;
         lastWasCR = true;
       }
     }
     if (lineNum > 1) {
       report += " (at line $lineNum, character ${offset - lineStart + 1})\n";
     } else {
       report += " (at character ${offset + 1})\n";
     }
     int lineEnd = DDC$RT.cast(source.length, dynamic, int, "CastGeneral",
         """line 141, column 19 of dart:core/exceptions.dart: """,
         source.length is int, true);
     for (int i = offset; i < source.length; i++) {
-      int char = ((__x19) => DDC$RT.cast(__x19, dynamic, int, "CastGeneral",
-          """line 143, column 18 of dart:core/exceptions.dart: """,
-          __x19 is int, true))(source.codeUnitAt(i));
+      int char = ((__x5) => DDC$RT.cast(__x5, dynamic, int, "CastGeneral",
+          """line 143, column 18 of dart:core/exceptions.dart: """, __x5 is int,
+          true))(source.codeUnitAt(i));
       if (char == 0x0a || char == 0x0d) {
         lineEnd = i;
         break;
       }
     }
     int length = lineEnd - lineStart;
     int start = lineStart;
     int end = lineEnd;
     String prefix = "";
     String postfix = "";
     if (length > 78) {
       int index = offset - lineStart;
       if (index < 75) {
         end = start + 75;
         postfix = "...";
       } else if (end - offset < 75) {
         start = end - 75;
         prefix = "...";
       } else {
         start = offset - 36;
         end = offset + 36;
         prefix = postfix = "...";
       }
     }
-    String slice = ((__x20) => DDC$RT.cast(__x20, dynamic, String,
-        "CastGeneral", """line 171, column 20 of dart:core/exceptions.dart: """,
-        __x20 is String, true))(source.substring(start, end));
+    String slice = ((__x6) => DDC$RT.cast(__x6, dynamic, String, "CastGeneral",
+        """line 171, column 20 of dart:core/exceptions.dart: """,
+        __x6 is String, true))(source.substring(start, end));
     int markOffset = offset - start + prefix.length;
     return "$report$prefix$slice$postfix\n${" " * markOffset}^\n";
   }
 }
 class IntegerDivisionByZeroException implements Exception {
   const IntegerDivisionByZeroException();
   String toString() => "IntegerDivisionByZeroException";
 }