dart2js: get:hashCode methods to avoid Object.hashCode

pkg/compiler/lib/src/closure.dart

Index: pkg/compiler/lib/src/closure.dart
diff --git a/pkg/compiler/lib/src/closure.dart b/pkg/compiler/lib/src/closure.dart
index 6530a99ecec0e7d4cfe2574d105fe3a754e0bbbd..0cffc4097aeb42543b15d0835c69ae9d17c19cf6 100644
--- a/pkg/compiler/lib/src/closure.dart
+++ b/pkg/compiler/lib/src/closure.dart
@@ -255,6 +255,9 @@ class BoxLocal extends Local {
   final String name;
   final ExecutableElement executableContext;
 
+  final int hashCode = _nextHashCode = (_nextHashCode + 10007).toUnsigned(30);
+  static int _nextHashCode = 0;
+
   BoxLocal(this.name, this.executableContext);
 
   String toString() => 'BoxLocal($name)';
@@ -326,7 +329,7 @@ class BoxFieldElement extends ElementX
 /// A local variable used encode the direct (uncaptured) references to [this].
 class ThisLocal extends Local {
   final ExecutableElement executableContext;
-  final hashCode = ++ElementX.elementHashCode;
+  final hashCode = ElementX.newHashCode();
 
   ThisLocal(this.executableContext);