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

pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
index 87203a2b1e5b13c412e29782d33cdc19f7a66513..40cb8ff9451bde2917fe6037de180ee30269e2f0 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
@@ -219,10 +219,24 @@ class IrBuilderDelimitedState {
   final ExecutableElement currentElement;
 
   final ir.Continuation returnContinuation = new ir.Continuation.retrn();
+  ir.Parameter _thisParameter;
 
   final List<ir.Definition> functionParameters = <ir.Definition>[];
 
   IrBuilderDelimitedState(this.constantSystem, this.currentElement);
+
+  ir.Parameter get thisParameter => _thisParameter;
+  void set thisParameter(ir.Parameter value) {
+    assert(_thisParameter == null);
+    _thisParameter = value;
+  }
+}
+
+class ThisParameterLocal implements Local {
+  final ExecutableElement executableContext;
+  ThisParameterLocal(this.executableContext);
+  String get name => 'this';
+  toString() => 'ThisParameterLocal($executableContext)';
 }
 
 /// A factory for building the cps IR.
@@ -296,6 +310,7 @@ abstract class IrBuilder {
   /// Add the given function parameter to the IR, and bind it in the environment
   /// or put it in its box, if necessary.
   void _createFunctionParameter(ParameterElement parameterElement);
+  void _createThisParameter();
 
   /// Creates an access to the receiver from the current (or enclosing) method.
   ///
@@ -402,6 +417,7 @@ abstract class IrBuilder {
   List<ir.Primitive> buildFunctionHeader(Iterable<ParameterElement> parameters,
                                         {ClosureScope closureScope,
                                          ClosureEnvironment env}) {
+    _createThisParameter();
     _enterClosureEnvironment(env);
     _enterScope(closureScope);
     parameters.forEach(_createFunctionParameter);
@@ -658,11 +674,11 @@ abstract class IrBuilder {
           message: "Local constants for abstract method $element: "
                    "${state.localConstants}"));
       return new ir.FunctionDefinition.abstract(
-                element, state.functionParameters, defaults);
+          element, state.thisParameter, state.functionParameters, defaults);
     } else {
       ir.RunnableBody body = makeRunnableBody();
       return new ir.FunctionDefinition(
-          element, state.functionParameters, body,
+          element, state.thisParameter, state.functionParameters, body,
           state.localConstants, defaults);
     }
   }
@@ -686,7 +702,7 @@ abstract class IrBuilder {
     FunctionElement element = state.currentElement;
     ir.RunnableBody body = makeRunnableBody();
     return new ir.ConstructorDefinition(
-        element, state.functionParameters, body, initializers,
+        element, state.thisParameter, state.functionParameters, body, initializers,
         state.localConstants, defaults);
   }
 
@@ -2037,6 +2053,15 @@ class DartIrBuilder extends IrBuilder {
     }
   }
 
+  void _createThisParameter() {
+    if (state.currentElement.isGenerativeConstructor ||
+        !(Elements.isStaticOrTopLevel(state.currentElement) ||
+          state.currentElement.isLocal)) {
+      state.thisParameter =
+          new ir.Parameter(new ThisParameterLocal(state.currentElement));
+    }
+  }
+
   void declareLocalVariable(LocalVariableElement variableElement,
                             {ir.Primitive initialValue}) {
     assert(isOpen);
@@ -2102,9 +2127,7 @@ class DartIrBuilder extends IrBuilder {
   }
 
   ir.Primitive buildThis() {
-    ir.Primitive thisPrim = new ir.This();
-    add(new ir.LetPrim(thisPrim));
-    return thisPrim;
+    return state.thisParameter;
   }
 
   ir.Primitive buildSuperInvocation(Element target,
@@ -2152,8 +2175,7 @@ class JsIrBuilder extends IrBuilder {
     if (env == null) return;
 
     // Obtain a reference to the function object (this).
-    ir.Primitive thisPrim = new ir.This();
-    add(new ir.LetPrim(thisPrim));
+    ir.Parameter thisPrim = state.thisParameter;
 
     // Obtain access to the free variables.
     env.freeVariables.forEach((Local local, ClosureLocation location) {
@@ -2216,6 +2238,13 @@ class JsIrBuilder extends IrBuilder {
     }
   }
 
+  void _createThisParameter() {
+    if (Element.isStaticOrTopLevel(state.currentElement)) return;
+    if (state.currentElement.isLocal) return;
+    state.thisParameter =
+        new ir.Parameter(new ThisParameterLocal(state.currentElement));
+  }
+
   void declareLocalVariable(LocalElement variableElement,
                             {ir.Primitive initialValue}) {
     assert(isOpen);
@@ -2324,9 +2353,7 @@ class JsIrBuilder extends IrBuilder {
 
   ir.Primitive buildThis() {
     if (jsState.receiver != null) return jsState.receiver;
-    ir.Primitive thisPrim = new ir.This();
-    add(new ir.LetPrim(thisPrim));
-    return thisPrim;
+    return state.thisParameter;
   }
 
   ir.Primitive buildSuperInvocation(Element target,
@@ -2370,6 +2397,7 @@ class JsIrBuilder extends IrBuilder {
   /// instead of being created in the header.
   void buildConstructorBodyHeader(Iterable<Local> parameters,
                                   ClosureScope closureScope) {
+    _createThisParameter();
     for (Local param in parameters) {
       ir.Parameter parameter = createLocalParameter(param);
       state.functionParameters.add(parameter);