improve DDC's type checks

pkg/dev_compiler/tool/input_sdk/private/js_helper.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart._js_helper;
 
 import 'dart:collection';
 
 import 'dart:_debugger' show stackTraceMapper;
 
@@ skipping 762 matching lines @@
 // When they are, remove the 'Implementation' here.
 
 /** Thrown by type assertions that fail. */
 class TypeErrorImplementation extends Error implements TypeError {
   final String message;
 
   /**
    * Normal type error caused by a failed subtype test.
    */
   // TODO(sra): Include [value] in message.
-  TypeErrorImplementation(Object value, Object actualType, Object expectedType)
+  TypeErrorImplementation(Object value, Object actualType, Object expectedType,
+      bool strongModeError)
       : message = "Type '${actualType}' is not a subtype "
-            "of type '${expectedType}'";
+            "of type '${expectedType}'" +
+            (strongModeError ? "" : " in strong mode");
 
   TypeErrorImplementation.fromMessage(String this.message);
 
   String toString() => message;
 }
 
 /** Thrown by the 'as' operator if the cast isn't valid. */
 class CastErrorImplementation extends Error implements CastError {
   // TODO(lrn): Rename to CastError (and move implementation into core).
   final String message;
 
   /**
    * Normal cast error caused by a failed type cast.
    */
   // TODO(sra): Include [value] in message.
-  CastErrorImplementation(Object value, Object actualType, Object expectedType)
+  CastErrorImplementation(Object value, Object actualType, Object expectedType,
+      bool strongModeError)
       : message = "CastError: Casting value of type '$actualType' to"
-            " incompatible type '$expectedType'";
+            " type '$expectedType' which is incompatible" +
+            (strongModeError ? "" : " in strong mode");
 
   String toString() => message;
 }
 
-/// Thrown by type assertions that fail in strong mode that would have passed in
-/// standard Dart.
-class StrongModeTypeError extends Error implements TypeError, StrongModeError {
-  final String message;
-  // TODO(sra): Include [value] in message.
-  StrongModeTypeError(Object value, Object actualType, Object expectedType)
-      : message = "Type '${actualType}' is not a subtype "
-            "of type '${expectedType}' in strong mode";
-  String toString() => message;
-}
-
-/// Thrown by casts that fail in strong mode that would have passed in standard
-/// Dart.
-class StrongModeCastError extends Error implements CastError, StrongModeError {
-  final String message;
-  // TODO(sra): Include [value] in message.
-  StrongModeCastError(Object value, Object actualType, Object expectedType)
-      : message = "CastError: Casting value of type '$actualType' to"
-            " type '$expectedType' which is incompatible in strong mode";
-  String toString() => message;
-}
-
 /// Used for Strong-mode errors other than type assertions and casts.
-class StrongModeErrorImplementation extends Error implements StrongModeError {
+// TODO(jmesserly): remove this: https://github.com/dart-lang/sdk/issues/30095
+class StrongModeErrorImplementation extends Error {
   final String message;
   StrongModeErrorImplementation(this.message);
   String toString() => message;
 }
 
 class FallThroughErrorImplementation extends FallThroughError {
   FallThroughErrorImplementation();
   String toString() => "Switch case fall-through.";
 }
 
@@ skipping 72 matching lines @@
     if (dart.polyfill(object)) {
       dart.applyAllExtensions(object);
     }
   } catch (e) {
     // This may fail due to cross-origin errors.  In that case, we shouldn't
     // need to polyfill as we can't get objects from that frame.
 
     // TODO(vsm): Detect this more robustly - ideally before we try to polyfill.
   }
 }