Rationalize coercions

test/dart_codegen/expect/async/async_error.dart

 part of dart.async;
  _invokeErrorHandler(Function errorHandler, Object error, StackTrace stackTrace) {
   if (errorHandler is ZoneBinaryCallback) {
     return errorHandler(error, stackTrace);
     }
    else {
     return errorHandler(error);
     }
   }
  Function _registerErrorHandler(Function errorHandler, Zone zone) {
   if (errorHandler is ZoneBinaryCallback) {
     return zone.registerBinaryCallback(errorHandler);
     }
    else {
-    return zone.registerUnaryCallback(DEVC$RT.cast(errorHandler, Function, __t0, "CastGeneral", """line 20, column 39 of dart:async/async_error.dart: """, errorHandler is __t0, true));
+    return zone.registerUnaryCallback(DEVC$RT.cast(errorHandler, Function, __t0, "ImplicitCast", """line 20, column 39 of dart:async/async_error.dart: """, errorHandler is __t0, true));
     }
   }
  class _UncaughtAsyncError extends AsyncError {_UncaughtAsyncError(error, StackTrace stackTrace) : super(error, _getBestStackTrace(error, stackTrace));
  static StackTrace _getBestStackTrace(error, StackTrace stackTrace) {
   if (stackTrace != null) return stackTrace;
    if (error is Error) {
-    return DEVC$RT.cast(error.stackTrace, dynamic, StackTrace, "CastGeneral", """line 31, column 14 of dart:async/async_error.dart: """, error.stackTrace is StackTrace, true);
+    return DEVC$RT.cast(error.stackTrace, dynamic, StackTrace, "DynamicCast", """line 31, column 14 of dart:async/async_error.dart: """, error.stackTrace is StackTrace, true);
     }
    return null;
   }
  String toString() {
   String result = "Uncaught Error: ${error}";
    if (stackTrace != null) {
     result += "\nStack Trace:\n$stackTrace";
     }
    return result;
   }
 }
  typedef dynamic __t0(dynamic __u1);