Use an explicit 'this' parameter instead of 'This' nodes.

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

Index: pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
index 8017ba65f47139a2866f32c6084ffe7b5592055c..841da5537dd12b301610f92198d15efeb9210c96 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
@@ -67,6 +67,7 @@ class Builder implements cps_ir.Visitor<Node> {
   List<cps_ir.Continuation> safeForInlining = <cps_ir.Continuation>[];
 
   ExecutableElement currentElement;
+  cps_ir.Parameter thisParameter;
   cps_ir.Continuation returnContinuation;
 
   Builder parent;
@@ -120,7 +121,10 @@ class Builder implements cps_ir.Visitor<Node> {
   /// referred to by [reference].
   /// This increments the reference count for the given variable, so the
   /// returned expression must be used in the tree.
-  VariableUse getVariableUse(cps_ir.Reference<cps_ir.Primitive> reference) {
+  Expression getVariableUse(cps_ir.Reference<cps_ir.Primitive> reference) {
+    if (reference.definition == thisParameter) {
+      return new This();
+    }
     Variable variable = getVariable(reference.definition);
     if (variable == null) {
       // Note: this may fail because you forgot to implement a visit-function
@@ -170,6 +174,7 @@ class Builder implements cps_ir.Visitor<Node> {
 
   FunctionDefinition buildFunction(cps_ir.FunctionDefinition node) {
     currentElement = node.element;
+    thisParameter = node.thisParameter;
     List<Variable> parameters =
         node.parameters.map(addFunctionParameter).toList();
     Statement body;
@@ -185,6 +190,7 @@ class Builder implements cps_ir.Visitor<Node> {
 
   ConstructorDefinition buildConstructor(cps_ir.ConstructorDefinition node) {
     currentElement = node.element;
+    thisParameter = node.thisParameter;
     List<Variable> parameters =
         node.parameters.map(addFunctionParameter).toList();
     List<Initializer> initializers;
@@ -575,10 +581,6 @@ class Builder implements cps_ir.Visitor<Node> {
     return new Constant(node.expression);
   }
 
-  Expression visitThis(cps_ir.This node) {
-    return new This();
-  }
-
   Expression visitReifyTypeVar(cps_ir.ReifyTypeVar node) {
     return new ReifyTypeVar(node.typeVariable);
   }