Added VariableUse expression to tree IR.

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
index 721f99098cdadb054f098ff958a89c7326a42961..61f0a3121de4010b2290fd73e3800ca95af61d52 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
@@ -37,8 +37,8 @@ part of tree_ir.optimization;
  * environment and the definition is recursively processed (in the
  * new environment at the use site) before being propagated.
  *
- * See [visitVariable] for the implementation of the heuristic for propagating
- * a definition.
+ * See [visitVariableUse] for the implementation of the heuristic for
+ * propagating a definition.
  *
  *
  * IF-TO-CONDITIONAL CONVERSION:
@@ -165,9 +165,10 @@ class StatementRewriter extends Visitor<Statement, Expression> with PassMixin {
 
   Expression visitExpression(Expression e) => e.processed ? e : e.accept(this);
 
-  Expression visitVariable(Variable node) {
+  @override
+  Expression visitVariableUse(VariableUse node) {
     // Propagate constant to use site.
-    Expression constant = constantEnvironment[node];
+    Expression constant = constantEnvironment[node.variable];
     if (constant != null) return constant;
 
     // Propagate a variable's definition to its use site if:
@@ -176,10 +177,11 @@ class StatementRewriter extends Visitor<Statement, Expression> with PassMixin {
     // 2.  It was the most recent expression evaluated so that we do not
     //     reorder expressions with side effects.
     if (!environment.isEmpty &&
-        environment.last.variable == node &&
+        environment.last.variable == node.variable &&
         environment.last.hasExactlyOneUse) {
       return visitExpression(environment.removeLast().definition);
     }
+
     // If the definition could not be propagated, leave the variable use.
     return node;
   }
@@ -193,7 +195,7 @@ class StatementRewriter extends Visitor<Statement, Expression> with PassMixin {
     return exp is Constant ||
            exp is This ||
            exp is ReifyTypeVar ||
-           exp is Variable && constantEnvironment.containsKey(exp);
+           exp is VariableUse && constantEnvironment.containsKey(exp.variable);
   }
 
   Statement visitAssign(Assign node) {
@@ -558,8 +560,8 @@ class StatementRewriter extends Visitor<Statement, Expression> with PassMixin {
   /// If non-null is returned, the caller must discard [e1] and [e2] and use
   /// the resulting expression in the tree.
   static Expression combineExpressions(Expression e1, Expression e2) {
-    if (e1 is Variable && e1 == e2) {
-      --e1.readCount; // Two references become one.
+    if (e1 is VariableUse && e2 is VariableUse && e1.variable == e2.variable) {
+      --e1.variable.readCount; // Two references become one.
       return e1;
     }
     if (e1 is Constant && e2 is Constant && e1.value == e2.value) {