Introduce DartType.unaliased to avoid use of Resolution in the backend.

pkg/compiler/lib/src/js_backend/runtime_types.dart

Index: pkg/compiler/lib/src/js_backend/runtime_types.dart
diff --git a/pkg/compiler/lib/src/js_backend/runtime_types.dart b/pkg/compiler/lib/src/js_backend/runtime_types.dart
index 8e31d939df334c3b1e8768cb38c9473d42535c83..274f6850a18c632bc5dbad2b4890e0bca2a6f066 100644
--- a/pkg/compiler/lib/src/js_backend/runtime_types.dart
+++ b/pkg/compiler/lib/src/js_backend/runtime_types.dart
@@ -898,7 +898,7 @@ class TypeRepresentationGenerator implements DartTypeVisitor {
 
   visitTypedefType(TypedefType type, _) {
     bool shouldEncode = shouldEncodeTypedef(type);
-    DartType unaliasedType = type.unalias(compiler.resolution);
+    DartType unaliasedType = type.unaliased;
     if (shouldEncode) {
       jsAst.ObjectInitializer initializer = unaliasedType.accept(this, null);
       // We have to encode the aliased type.
@@ -968,7 +968,7 @@ class ArgumentCollector extends DartTypeVisitor {
   }
 
   visitTypedefType(TypedefType type, bool isTypeArgument) {
-    type.unalias(backend.resolution).accept(this, isTypeArgument);
+    type.unaliased.accept(this, isTypeArgument);
   }
 
   visitInterfaceType(InterfaceType type, bool isTypeArgument) {
@@ -1000,7 +1000,7 @@ class FunctionArgumentCollector extends DartTypeVisitor {
   }
 
   visitTypedefType(TypedefType type, bool inFunctionType) {
-    type.unalias(backend.resolution).accept(this, inFunctionType);
+    type.unaliased.accept(this, inFunctionType);
   }
 
   visitInterfaceType(InterfaceType type, bool inFunctionType) {