Added VariableUse expression to tree IR.

pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart

Index: pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
index 4312130deaa2b4bbc058c94fd8199a04fd8c6eff..10176e2acf2bef84def8687ade5ef7d5d5d2b155 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
@@ -84,9 +84,20 @@ class Label {
 }
 
 /**
- * Variables are [Expression]s.
+ * A local variable in the tree IR.
+ *
+ * All tree IR variables are mutable, and may in Dart-mode be referenced inside
+ * nested functions.
+ *
+ * To use a variable as an expression, reference it from a [VariableUse], with
+ * one [VariableUse] per expression.
+ *
+ * [Variable]s are reference counted. The node constructors [VariableUse],
+ * [Assign], [FunctionDefinition] automatically update the reference count
+ * for their variables, but when transforming the tree, the transformer is
+ * responsible for updating reference counts.
  */
-class Variable extends Expression {
+class Variable extends Node {
   /// Function that declares this variable.
   ExecutableElement host;
 
@@ -94,6 +105,7 @@ class Variable extends Expression {
   /// Different variables may have the same entity. May be null.
   Entity element;
 
+  /// Number of places where this variable occurs in a [VariableUse].
   int readCount = 0;
 
   /// Number of places where this variable occurs as:
@@ -105,9 +117,21 @@ class Variable extends Expression {
   Variable(this.host, this.element) {
     assert(host != null);
   }
+}
+
+/// Read the value of a variable.
+class VariableUse extends Expression {
+  Variable variable;
+
+  /// Creates a use of [variable] and updates its `readCount`.
+  VariableUse(this.variable) {
+    variable.readCount++;
+  }
 
-  accept(ExpressionVisitor visitor) => visitor.visitVariable(this);
-  accept1(ExpressionVisitor1 visitor, arg) => visitor.visitVariable(this, arg);
+  accept(ExpressionVisitor visitor) => visitor.visitVariableUse(this);
+  accept1(ExpressionVisitor1 visitor, arg) {
+    return visitor.visitVariableUse(this, arg);
+  }
 }
 
 /**
@@ -498,6 +522,7 @@ class Assign extends Statement {
   /// Variable declarations themselves are hoisted to function level.
   bool isDeclaration;
 
+  /// Creates an assignment to [variable] and updates its `writeCount`.
   Assign(this.variable, this.definition, this.next,
          { this.isDeclaration: false }) {
     variable.writeCount++;
@@ -602,8 +627,13 @@ class FunctionDefinition extends Node implements ExecutableDefinition {
   final List<ConstDeclaration> localConstants;
   final List<ConstantExpression> defaultParameterValues;
 
+  /// Creates a function definition and updates `writeCount` for [parameters].
   FunctionDefinition(this.element, this.parameters, this.body,
-      this.localConstants, this.defaultParameterValues);
+      this.localConstants, this.defaultParameterValues) {
+    for (Variable param in parameters) {
+      param.writeCount++; // Being a parameter counts as a write.
+    }
+  }
 
   /// Returns `true` if this function is abstract.
   ///
@@ -700,7 +730,7 @@ class SetField extends Statement implements JsSpecificNode {
 
 abstract class ExpressionVisitor<E> {
   E visitExpression(Expression e) => e.accept(this);
-  E visitVariable(Variable node);
+  E visitVariableUse(VariableUse node);
   E visitInvokeStatic(InvokeStatic node);
   E visitInvokeMethod(InvokeMethod node);
   E visitInvokeMethodDirectly(InvokeMethodDirectly node);
@@ -725,7 +755,7 @@ abstract class ExpressionVisitor<E> {
 
 abstract class ExpressionVisitor1<E, A> {
   E visitExpression(Expression e, A arg) => e.accept1(this, arg);
-  E visitVariable(Variable node, A arg);
+  E visitVariableUse(VariableUse node, A arg);
   E visitInvokeStatic(InvokeStatic node, A arg);
   E visitInvokeMethod(InvokeMethod node, A arg);
   E visitInvokeMethodDirectly(InvokeMethodDirectly node, A arg);
@@ -792,11 +822,16 @@ abstract class Visitor1<S, E, A> implements ExpressionVisitor1<E, A>,
 
 class RecursiveVisitor extends Visitor {
   visitFunctionDefinition(FunctionDefinition node) {
+    node.parameters.forEach(visitVariable);
     visitStatement(node.body);
   }
 
   visitVariable(Variable node) {}
 
+  visitVariableUse(VariableUse node) {
+    visitVariable(node.variable);
+  }
+
   visitInvokeStatic(InvokeStatic node) {
     node.arguments.forEach(visitExpression);
   }