Refactor strong mode to use standard Analyzer errors

pkg/analyzer/lib/src/generated/type_system.dart

Index: pkg/analyzer/lib/src/generated/type_system.dart
diff --git a/pkg/analyzer/lib/src/generated/type_system.dart b/pkg/analyzer/lib/src/generated/type_system.dart
index 7e4103d9e10266574e2830246831c51f9d151ecc..229b8e484a8db0b4c7b34a902ce33d218f126b56 100644
--- a/pkg/analyzer/lib/src/generated/type_system.dart
+++ b/pkg/analyzer/lib/src/generated/type_system.dart
@@ -12,7 +12,8 @@ import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/type.dart';
-import 'package:analyzer/src/generated/engine.dart' show AnalysisContext;
+import 'package:analyzer/src/generated/engine.dart'
+    show AnalysisContext, AnalysisOptionsImpl;
 import 'package:analyzer/src/generated/resolver.dart' show TypeProvider;
 import 'package:analyzer/src/generated/utilities_dart.dart';
 
@@ -23,6 +24,15 @@ typedef bool _GuardedSubtypeChecker<T>(T t1, T t2, Set<Element> visited);
  * https://github.com/dart-lang/dev_compiler/blob/master/STRONG_MODE.md
  */
 class StrongTypeSystemImpl extends TypeSystem {
+  /**
+   * True if implicit casts should be allowed, otherwise false.
+   *
+   * This affects the behavior of [isAssignableTo].
+   */
+  final bool implicitCasts;
+
+  StrongTypeSystemImpl({this.implicitCasts: true});
+
   bool anyParameterType(FunctionType ft, bool predicate(DartType t)) {
     return ft.parameters.any((p) => predicate(p.type));
   }
@@ -309,17 +319,16 @@ class StrongTypeSystemImpl extends TypeSystem {
       return false;
     }
 
-    // If the subtype relation goes the other way, allow the implicit downcast.
-    // TODO(leafp): Emit warnings and hints for these in some way.
-    // TODO(leafp): Consider adding a flag to disable these?  Or just rely on
-    //   --warnings-as-errors?
-    if (isSubtypeOf(toType, fromType) || toType.isAssignableTo(fromType)) {
-      // TODO(leafp): error if type is known to be exact (literal,
-      //  instance creation).
-      // TODO(leafp): Warn on composite downcast.
-      // TODO(leafp): hint on object/dynamic downcast.
-      // TODO(leafp): Consider allowing assignment casts.
-      return true;
+    if (implicitCasts) {
+      // If the subtype relation goes the other way, allow the implicit
+      // downcast.
+      if (isSubtypeOf(toType, fromType) || toType.isAssignableTo(fromType)) {
+        // TODO(leafp,jmesserly): we emit warnings/hints for these in
+        // src/task/strong/checker.dart, which is a bit inconsistent. That
+        // code should be handled into places that use isAssignableTo, such as
+        // ErrorVerifier.
+        return true;
+      }
     }
 
     return false;
@@ -1180,8 +1189,9 @@ abstract class TypeSystem {
    * Create either a strong mode or regular type system based on context.
    */
   static TypeSystem create(AnalysisContext context) {
-    return (context.analysisOptions.strongMode)
-        ? new StrongTypeSystemImpl()
+    var options = context.analysisOptions as AnalysisOptionsImpl;
+    return options.strongMode
+        ? new StrongTypeSystemImpl(implicitCasts: options.implicitCasts)
         : new TypeSystemImpl();
   }
 }