dart2js cps: Inline constructor bodies that are only called once.

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

Index: pkg/compiler/lib/src/cps_ir/inline.dart
diff --git a/pkg/compiler/lib/src/cps_ir/inline.dart b/pkg/compiler/lib/src/cps_ir/inline.dart
index 08375a2468d6e1fbf3055306a47bbcbe1b24660b..076e809581579b6345116c7330d78b097dad0e82 100644
--- a/pkg/compiler/lib/src/cps_ir/inline.dart
+++ b/pkg/compiler/lib/src/cps_ir/inline.dart
@@ -185,6 +185,11 @@ class Inliner implements Pass {
   Inliner(this.functionCompiler);
 
   bool isCalledOnce(Element element) {
+    if (element is ConstructorBodyElement) {
+      ClassElement class_ = element.enclosingClass;
+      return !functionCompiler.compiler.world.hasAnyStrictSubclass(class_) &&
+          class_.constructors.tail?.isEmpty ?? false;
+    }
     return functionCompiler.compiler.typesTask.typesInferrer.isCalledOnce(
         element);
   }