Added VariableUse expression to tree IR.

pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart

Index: pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart
diff --git a/pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart b/pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart
index 277966315b3c6ee24c63c05a598664cff5034e28..dca7038f2d703f9bf736775fceb77a07a2e26d50 100644
--- a/pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart
+++ b/pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart
@@ -536,7 +536,7 @@ class ASTEmitter
     }
 
     // Emit a variable declaration if we are required to do so.
-    // This is to ensure that a fresh closure variable is created.
+    // For captured variables, this ensures that a fresh variable is created.
     if (stmt.isDeclaration) {
       assert(isFirstOccurrence);
       assert(isDeclaredHere);
@@ -831,6 +831,11 @@ class ASTEmitter
   }
 
   @override
+  Expression visitVariableUse(tree.VariableUse exp,
+                                     BuilderContext<Statement> context) {

[Kevin Millikin (Google), 2015/02/26 12:43:17]

Indentation is messed up.

[asgerf, 2015/02/27 12:05:19]

Done.

+    return visitVariable(exp.variable, context);
+  }
+
   Expression visitVariable(tree.Variable exp,

[Kevin Millikin (Google), 2015/02/26 12:43:17]

I'd probably get rid of this visitVariable method --- it's not required by the
superclass tree.Visitor1, and it isn't actually called for all Variables in the
input (it's not called for FunctionDefinition parameters).

Instead, I'd put a method on the BuilderContext and call it explicitly in the
two places where visitVariable is currently called.

[asgerf, 2015/02/27 12:05:19]

Done.

                            BuilderContext<Statement> context) {
     return new Identifier(context.getVariableName(exp))
@@ -1233,12 +1238,15 @@ class UnshadowParameters extends tree.RecursiveVisitor {
         tree.Variable newParam = new tree.Variable(definition.element,
             param.element);
         definition.parameters[i] = newParam;
-        definition.body = new tree.Assign(param, newParam, definition.body);
+        definition.body = new tree.Assign(param, new tree.VariableUse(newParam),
+            definition.body);
         newParam.writeCount = 1; // Being a parameter counts as a write.
+        param.writeCount--; // Not a parameter anymore.

[asgerf, 2015/02/25 12:40:30]

This was technically a bug, although the reference counts are not used after
this phase, so it never manifested.

       }
     }
   }
 
+  @override
   visitVariable(tree.Variable variable) {
     if (shadowedParameters.contains(variable)) {
       hasShadowedUse.add(variable);