Be smarter about when to disable global type inference.

dart/sdk/lib/_internal/compiler/implementation/compiler.dart

Index: dart/sdk/lib/_internal/compiler/implementation/compiler.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
index c850caa3ae3367d60e30319a08121a9cf29b1a8a..446e36a8015141ff424397f3ed3b27c85e10d910 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -453,6 +453,9 @@ abstract class Compiler implements DiagnosticListener {
 
   bool hasCrashed = false;
 
+  /// Set by the backend if real reflection is detected in use of dart:mirrors.
+  bool disableTypeInferenceForMirrors = false;
+
   Compiler({this.tracer: const Tracer(),
             this.enableTypeAssertions: false,
             this.enableUserAssertions: false,
@@ -530,7 +533,9 @@ abstract class Compiler implements DiagnosticListener {
 
   bool get compileAll => false;
 
-  bool get disableTypeInference => disableTypeInferenceFlag || mirrorsEnabled;
+  bool get disableTypeInference {
+    return disableTypeInferenceFlag || disableTypeInferenceForMirrors;
+  }
 
   int getNextFreeClassId() => nextFreeClassId++;