Add one-char string table for faster String.charAt to the VM.

runtime/vm/snapshot_test.cc

Index: runtime/vm/snapshot_test.cc
===================================================================
--- runtime/vm/snapshot_test.cc	(revision 14902)
+++ runtime/vm/snapshot_test.cc	(working copy)
@@ -1840,11 +1840,9 @@
       for (int i = 0; i < kArrayLength; i++) {
         Dart_CObject* element = root->value.as_array.values[i];
         if ((i % 2) == 0) {
-          EXPECT_EQ(root->value.as_array.values[0], element);
           EXPECT_EQ(Dart_CObject::kString, element->type);
           EXPECT_STREQ("A", element->value.as_string);
         } else {
-          EXPECT_EQ(root->value.as_array.values[1], element);
           EXPECT_EQ(Dart_CObject::kDouble, element->type);
           EXPECT_STREQ(2.72, element->value.as_double);
         }
@@ -2015,11 +2013,9 @@
       for (int i = 0; i < kArrayLength; i++) {
         Dart_CObject* element = root->value.as_array.values[i];
         if ((i % 2) == 0) {
-          EXPECT_EQ(root->value.as_array.values[0], element);
           EXPECT_EQ(Dart_CObject::kString, element->type);
           EXPECT_STREQ("A", element->value.as_string);
         } else {
-          EXPECT_EQ(root->value.as_array.values[1], element);
           EXPECT_EQ(Dart_CObject::kDouble, element->type);
           EXPECT_STREQ(2.72, element->value.as_double);
         }