Add missing return statement.

lib/compiler/implementation/ssa/codegen.dart

 // Copyright (c) 2012, 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.
 
 class SsaCodeGeneratorTask extends CompilerTask {
   SsaCodeGeneratorTask(Compiler compiler) : super(compiler);
   String get name() => 'SSA code generator';
 
 
   String buildJavaScriptFunction(FunctionElement element,
@@ skipping 561 matching lines @@
         // Foo a = foo();
         // print(a);
         //
         // In checked mode will turn into:
         // print(FooTypeCheck(foo()));
         visit(argument, expectedPrecedenceForArgument);
       } else if (isGenerateAtUseSite(input)) {
         // If [argument] cannot be generated at use site, but [input]
         // can, use the temporary of [argument]. A code motion
         // invariant instruction does not have a temporary, so we just
-        // 
+        //
         // For example:
         // Foo a = foo();
         // print(a);
         // print(a);
         //
         // In checked mode will turn into:
         // var a = FooTypeCheck(foo());
         // print(a);
         // print(a);
         //
@@ skipping 86 matching lines @@
     }
     return true;
   }
 
   bool visitSequenceInfo(HStatementSequenceInformation info) {
     return false;
   }
 
   bool visitSubGraphInfo(HSubGraphBlockInformation info) {
     visitSubGraph(info.subGraph);
-    // A [HSubGraphBlockInformation] is always part of another block
-    // information structure, so it doesn't have a joinBlock.
+    return true;
   }
 
   bool visitSubExpressionInfo(HSubExpressionBlockInformation info) {
     return false;
   }
 
   bool visitAndOrInfo(HAndOrBlockInformation info) {
     return false;
   }
 
@@ skipping 1893 matching lines @@
       startBailoutSwitch();
     }
   }
 
   void endLabeledBlock(HLabeledBlockInformation labeledBlockInfo) {
     if (labeledBlockInfo.body.start.hasGuards()) {
       endBailoutSwitch();
     }
   }
 }