Implement references to "this" in the new IR.

sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart

Index: sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart b/sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart
index 077829d9c8475b0c05d1c1614e5565876825ec57..e8f61d009c3b54fc390a098d0bfbc1c422e14394 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart
@@ -174,6 +174,10 @@ class Constant extends Expression {
   accept(ExpressionVisitor visitor) => visitor.visitConstant(this);
 }
 
+class This extends Expression {
+  accept(Visitor visitor) => visitor.visitThis(this);
+}
+
 class LiteralList extends Expression {
   final GenericType type;
   final List<Expression> values;
@@ -421,6 +425,7 @@ abstract class ExpressionVisitor<E> {
   E visitInvokeConstructor(InvokeConstructor node);
   E visitConcatenateStrings(ConcatenateStrings node);
   E visitConstant(Constant node);
+  E visitThis(This node);
   E visitConditional(Conditional node);
   E visitLogicalOperator(LogicalOperator node);
   E visitNot(Not node);
@@ -808,6 +813,10 @@ class Builder extends ir.Visitor<Node> {
     return new Constant(node.value);
   }
 
+  Expression visitThis(ir.This node) {
+    return new This();
+  }
+
   Expression visitLiteralList(ir.LiteralList node) {
     return new LiteralList(
             node.type,
@@ -1155,6 +1164,10 @@ class StatementRewriter extends Visitor<Statement, Expression> {
     return node;
   }
 
+  Expression visitThis(This node) {
+    return node;
+  }
+
   Expression visitLiteralList(LiteralList node) {
     // Process values right-to-left, the opposite of evaluation order.
     for (int i = node.values.length - 1; i >= 0; --i) {
@@ -1667,6 +1680,10 @@ class LogicalRewriter extends Visitor<Statement, Expression> {
     return node;
   }
 
+  Expression visitThis(This node) {
+    return node;
+  }
+
   Expression visitNot(Not node) {
     return toBoolean(makeCondition(node.operand, false, liftNots: false));
   }