dart2js cps: Better fallthrough analysis and eliminate "return null".

tests/compiler/dart2js/js_backend_cps_ir_basic_test.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.
 
 // Tests for basic functionality.
 
 library basic_tests;
 
 import 'js_backend_cps_ir.dart';
 
@@ skipping 16 matching lines @@
 function() {
   var l = [1, 2, 3], m = P.LinkedHashMap_LinkedHashMap$_literal(["s", 1]);
   P.print("()");
   P.print("(true)");
   P.print("(1)");
   P.print("(" + H.S([1, 2, 3]) + ")");
   P.print("(" + H.S(P.LinkedHashMap_LinkedHashMap$_literal(["s", 1])) + ")");
   P.print("(1)");
   P.print("(" + H.S(l) + ")");
   P.print("(" + H.S(m) + ")");
-  return null;
 }"""),
   const TestEntry("""
 foo(a, [b = "b"]) => b;
 bar(a, {b: "b", c: "c"}) => c;
 main() {
   foo(0);
   foo(1, 2);
   bar(3);
   bar(4, b: 5);
   bar(6, c: 7);
   bar(8, b: 9, c: 10);
 }
 """,
 """
 function() {
   V.foo(0, "b");
   V.foo(1, 2);
   V.bar(3, "b", "c");
   V.bar(4, 5, "c");
   V.bar(6, "b", 7);
   V.bar(8, 9, 10);
-  return null;
 }"""),
   const TestEntry(
   """
 foo(a) {
   return a;
 }
 main() {
   var a = 10;
   var b = 1;
   var t;
   t = a;
   a = b;
   b = t;
   print(a);
   print(b);
   print(b);
   print(foo(a));
 }
   """,
   """
 function() {
   var a = 10, b = 1;
   P.print(b);
   P.print(a);
   P.print(a);
   P.print(V.foo(b));
-  return null;
 }"""),
   const TestEntry(
   """
 foo() { return 42; }
 main() { return foo(); }
   """,
   """function() {
   return V.foo();
 }"""),
   const TestEntry("main() {}"),
   const TestEntry("main() { return 42; }"),
   const TestEntry("main() { return; }", """
 function() {
-  return null;
 }"""),
   // Constructor invocation
   const TestEntry("""
 main() {
   print(new Set());
   print(new Set.from([1, 2, 3]));
 }""", r"""
 function() {
   P.print(P.LinkedHashSet_LinkedHashSet(null, null, null, null));
   P.print(P.LinkedHashSet_LinkedHashSet$from([1, 2, 3], null));
-  return null;
 }"""),
   // Call synthetic constructor.
   const TestEntry("""
 class C {}
 main() {
   print(new C());
 }"""),
   // Method invocation
   const TestEntry("""
 main() {
   print(new DateTime.now().isBefore(new DateTime.now()));
 }""", r"""
 function() {
   P.print(P.DateTime$now().isBefore$1(P.DateTime$now()));
-  return null;
 }"""),
   // Static calls
   const TestEntry("""
 foo() { print(42); }
 main() { foo(); }
 """, r"""
 function() {
   V.foo();
-  return null;
 }"""),
   // Static getters
   const TestEntry("""
 var foo = 42;
 main() { print(foo); }
 """, r"""
 function() {
   P.print($.foo);
-  return null;
 }"""),
   const TestEntry("""
 get foo { print(42); }
 main() { foo; }
 """, r"""
 function() {
   V.foo();
-  return null;
 }"""),
   // Static setters
   const TestEntry("""
 var foo = 0;
 main() { print(foo = 42); }
 """, r"""
 function() {
   var v0 = 42;
   $.foo = v0;
   P.print(v0);
-  return null;
 }"""),
   const TestEntry("""
 set foo(x) { print(x); }
 main() { foo = 42; }
 """, r"""
 function() {
   V.foo(42);
-  return null;
 }"""),
   // Assert
   const TestEntry("""
 foo() { print('X'); }
 main() {
   assert(true);
   assert(false);
   assert(foo());
   print('Done');
 }""", r"""
 function() {
   P.print("Done");
-  return null;
 }""")
 ];
 
 
 void main() {
   runTests(tests);
 }