Add a new implementation of HashMap that uses JS objects for its (multiple) hash tables.

runtime/vm/vm.gypi

Index: runtime/vm/vm.gypi
diff --git a/runtime/vm/vm.gypi b/runtime/vm/vm.gypi
index a1edd385ff151f80e7a6a3615a9f8e88ccf6fbd6..9fe4b83e0404e502dc0dc63ec032883f4c5741fc 100644
--- a/runtime/vm/vm.gypi
+++ b/runtime/vm/vm.gypi
@@ -10,6 +10,7 @@
     'corelib_cc_file': '<(SHARED_INTERMEDIATE_DIR)/corelib_gen.cc',
     'corelib_patch_cc_file': '<(SHARED_INTERMEDIATE_DIR)/corelib_patch_gen.cc',
     'collection_cc_file': '<(SHARED_INTERMEDIATE_DIR)/collection_gen.cc',
+    'collection_patch_cc_file': '<(SHARED_INTERMEDIATE_DIR)/collection_patch_gen.cc',
     'collection_dev_cc_file': '<(SHARED_INTERMEDIATE_DIR)/collection_dev_gen.cc',
     'crypto_cc_file': '<(SHARED_INTERMEDIATE_DIR)/crypto_gen.cc',
     'math_cc_file': '<(SHARED_INTERMEDIATE_DIR)/math_gen.cc',
@@ -97,6 +98,7 @@
         'generate_corelib_cc_file',
         'generate_corelib_patch_cc_file',
         'generate_collection_cc_file',
+        'generate_collection_patch_cc_file',
         'generate_collection_dev_cc_file',
         'generate_crypto_cc_file',
         'generate_math_cc_file',
@@ -116,6 +118,7 @@
       ],
       'includes': [
         '../lib/async_sources.gypi',
+        '../lib/collection_sources.gypi',
         '../lib/lib_sources.gypi',
         '../lib/isolate_sources.gypi',
         '../lib/math_sources.gypi',
@@ -131,6 +134,7 @@
         '<(corelib_cc_file)',
         '<(corelib_patch_cc_file)',
         '<(collection_cc_file)',
+        '<(collection_patch_cc_file)',
         '<(collection_dev_cc_file)',
         '<(crypto_cc_file)',
         '<(math_cc_file)',
@@ -157,6 +161,7 @@
       'type': 'static_library',
       'includes': [
         '../lib/async_sources.gypi',
+        '../lib/collection_sources.gypi',
         '../lib/lib_sources.gypi',
         '../lib/isolate_sources.gypi',
         '../lib/math_sources.gypi',
@@ -771,6 +776,44 @@
       ]
     },
     {
+      'target_name': 'generate_collection_patch_cc_file',
+      'type': 'none',
+      'includes': [
+        # Load the runtime implementation sources.
+        '../lib/collection_sources.gypi',
+      ],
+      'sources/': [
+        # Exclude all .[cc|h] files.
+        # This is only here for reference. Excludes happen after
+        # variable expansion, so the script has to do its own
+        # exclude processing of the sources being passed.
+        ['exclude', '\\.cc|h$'],
+      ],
+      'actions': [
+        {
+          'action_name': 'generate_collection_patch_cc',
+          'inputs': [
+            '../tools/create_string_literal.py',
+            '<(builtin_in_cc_file)',
+            '<@(_sources)',
+          ],
+          'outputs': [
+            '<(collection_patch_cc_file)',
+          ],
+          'action': [
+            'python',
+            'tools/create_string_literal.py',
+            '--output', '<(collection_patch_cc_file)',
+            '--input_cc', '<(builtin_in_cc_file)',
+            '--include', 'vm/bootstrap.h',
+            '--var_name', 'dart::Bootstrap::collection_patch_',
+            '<@(_sources)',
+          ],
+          'message': 'Generating ''<(collection_patch_cc_file)'' file.'
+        },
+      ]
+    },
+    {
       'target_name': 'generate_isolate_patch_cc_file',
       'type': 'none',
       'includes': [