Introduce per-isolate assert scopes and API to guard JS execution.

test/cctest/test-strings.cc

Index: test/cctest/test-strings.cc
diff --git a/test/cctest/test-strings.cc b/test/cctest/test-strings.cc
index 4b31e614d519dfdabe13d5008855104eeb1ca74b..129e6cf3d6263613b30cc43816f8e252e9c03583 100644
--- a/test/cctest/test-strings.cc
+++ b/test/cctest/test-strings.cc
@@ -661,7 +661,7 @@ void TestStringCharacterStream(BuildString build, int test_cases) {
   for (int i = 0; i < test_cases; i++) {
     printf("%d\n", i);
     HandleScope inner_scope(isolate);
-    AlwaysAllocateScope always_allocate;
+    AlwaysAllocateScope always_allocate(isolate);
     // Build flat version of cons string.
     Handle<String> flat_string = build(i, &data);
     ConsStringStats flat_string_stats;