don't run dart gen on JS patch code

test/dart_codegen/expect/core/errors.dart

 part of dart.core;
 
 class Error {
   Error();
   static String safeToString(Object object) {
     if (object is num || object is bool || null == object) {
       return object.toString();
     }
     if (object is String) {
       return _stringToSafeString(object);
     }
     return _objectToString(object);
   }
-  static String _stringToSafeString(String string) {
-    return ((__x14) => DDC$RT.cast(__x14, dynamic, String, "CastGeneral",
-        """line 89, column 12 of dart:core/errors.dart: """, __x14 is String,
-        true))(jsonEncodeNative(string));
-  }
-  static String _objectToString(Object object) {
-    return ((__x15) => DDC$RT.cast(__x15, dynamic, String, "CastGeneral",
-        """line 93, column 12 of dart:core/errors.dart: """, __x15 is String,
-        true))(Primitives.objectToString(object));
-  }
-  StackTrace get stackTrace => ((__x16) => DDC$RT.cast(__x16, dynamic,
-      StackTrace, "CastGeneral",
-      """line 96, column 32 of dart:core/errors.dart: """, __x16 is StackTrace,
-      true))(Primitives.extractStackTrace(this));
+  external static String _stringToSafeString(String string);
+  external static String _objectToString(Object object);
+  external StackTrace get stackTrace;
 }
 class AssertionError extends Error {}
 class TypeError extends AssertionError {}
 class CastError extends Error {}
 class NullThrownError extends Error {
   String toString() => "Throw of null.";
 }
 class ArgumentError extends Error {
   final bool _hasValue;
   final invalidValue;
@@ skipping 46 matching lines @@
       [String name, String message, int length]) = IndexError;
   static void checkValueInInterval(int value, int minValue, int maxValue,
       [String name, String message]) {
     if (value < minValue || value > maxValue) {
       throw new RangeError.range(value, minValue, maxValue, name, message);
     }
   }
   static void checkValidIndex(int index, var indexable,
       [String name, int length, String message]) {
     if (length == null) length = DDC$RT.cast(indexable.length, dynamic, int,
-        "CastGeneral", """line 285, column 34 of dart:core/errors.dart: """,
+        "CastGeneral", """line 281, column 34 of dart:core/errors.dart: """,
         indexable.length is int, true);
     if (index < 0 || index >= length) {
       if (name == null) name = "index";
       throw new RangeError.index(index, indexable, name, message, length);
     }
   }
   static void checkValidRange(int start, int end, int length,
       [String startName, String endName, String message]) {
     if (start < 0 || start > length) {
       if (startName == null) startName = "start";
@@ skipping 63 matching lines @@
   final Map<Symbol, dynamic> _namedArguments;
   final List _existingArgumentNames;
   NoSuchMethodError(Object receiver, Symbol memberName,
       List positionalArguments, Map<Symbol, dynamic> namedArguments,
       [List existingArgumentNames = null])
       : _receiver = receiver,
         _memberName = memberName,
         _arguments = positionalArguments,
         _namedArguments = namedArguments,
         _existingArgumentNames = existingArgumentNames;
-  String toString() {
-    StringBuffer sb = new StringBuffer();
-    int i = 0;
-    if (_arguments != null) {
-      for (; i < _arguments.length; i++) {
-        if (i > 0) {
-          sb.write(", ");
-        }
-        sb.write(Error.safeToString(_arguments[i]));
-      }
-    }
-    if (_namedArguments != null) {
-      _namedArguments.forEach((Symbol key, var value) {
-        if (i > 0) {
-          sb.write(", ");
-        }
-        sb.write(_symbolToString(key));
-        sb.write(": ");
-        sb.write(Error.safeToString(value));
-        i++;
-      });
-    }
-    if (_existingArgumentNames == null) {
-      return "NoSuchMethodError : method not found: '$_memberName'\n" "Receiver: ${Error.safeToString(_receiver)}\n" "Arguments: [$sb]";
-    } else {
-      String actualParameters = sb.toString();
-      sb = new StringBuffer();
-      for (int i = 0; i < _existingArgumentNames.length; i++) {
-        if (i > 0) {
-          sb.write(", ");
-        }
-        sb.write(_existingArgumentNames[i]);
-      }
-      String formalParameters = sb.toString();
-      return "NoSuchMethodError: incorrect number of arguments passed to " "method named '$_memberName'\n" "Receiver: ${Error.safeToString(_receiver)}\n" "Tried calling: $_memberName($actualParameters)\n" "Found: $_memberName($formalParameters)";
-    }
-  }
+  external String toString();
 }
 class UnsupportedError extends Error {
   final String message;
   UnsupportedError(this.message);
   String toString() => "Unsupported operation: $message";
 }
 class UnimplementedError extends Error implements UnsupportedError {
   final String message;
   UnimplementedError([String this.message]);
   String toString() => (this.message != null
@@ skipping 25 matching lines @@
   String toString() => "Stack Overflow";
   StackTrace get stackTrace => null;
 }
 class CyclicInitializationError extends Error {
   final String variableName;
   CyclicInitializationError([this.variableName]);
   String toString() => variableName == null
       ? "Reading static variable during its initialization"
       : "Reading static variable '$variableName' during its initialization";
 }