Add missing return statement.

lib/compiler/implementation/ssa/codegen.dart

Index: lib/compiler/implementation/ssa/codegen.dart
diff --git a/lib/compiler/implementation/ssa/codegen.dart b/lib/compiler/implementation/ssa/codegen.dart
index e473166d5ef7415cc60d39b70b63cf2777ab5065..ed51df7e9d1898ba3ed03941836a6aa2e32018d8 100644
--- a/lib/compiler/implementation/ssa/codegen.dart
+++ b/lib/compiler/implementation/ssa/codegen.dart
@@ -579,7 +579,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         // 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);
@@ -686,8 +686,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
   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) {