Fix a merge error that was included in r42429.

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library tree_ir_builder;
 
 import '../dart2jslib.dart' as dart2js;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
@@ skipping 95 matching lines @@
           CURRENT_ELEMENT_SPANNABLE,
           "Reference to ${reference.definition} has no register");
     }
     ++variable.readCount;
     return variable;
   }
 
   ExecutableDefinition build(cps_ir.ExecutableDefinition node) {
     if (node is cps_ir.FieldDefinition) {
       return buildField(node);
+    } else if (node is cps_ir.ConstructorDefinition) {
+      return buildConstructor(node);
     } else {
       assert(dart2js.invariant(
           CURRENT_ELEMENT_SPANNABLE,
           node is cps_ir.FunctionDefinition,
           message: 'expected FunctionDefinition or FieldDefinition, '
             ' found $node'));
       return buildFunction(node);
     }
-    return null;
   }
 
   FieldDefinition buildField(cps_ir.FieldDefinition node) {
     Statement body;
     if (node.hasInitializer) {
       currentElement = node.element;
       returnContinuation = node.body.returnContinuation;
 
       phiTempVar = new Variable(node.element, null);
 
@@ skipping 409 matching lines @@
     // visited.
     internalError(CURRENT_ELEMENT_SPANNABLE, 'Unexpected IR node: $node.');
     return null;
   }
 
   Expression visitIsTrue(cps_ir.IsTrue node) {
     return getVariableReference(node.value);
   }
 }