Add support for building the Dart VM for Android OS.

runtime/dart-runtime.gyp

Index: runtime/dart-runtime.gyp
diff --git a/runtime/dart-runtime.gyp b/runtime/dart-runtime.gyp
index a2d79e04e22847cd57968cd35c1131c9cabc4c88..0814073cbab42ef40d9779b6200c4a6b938dd18b 100644
--- a/runtime/dart-runtime.gyp
+++ b/runtime/dart-runtime.gyp
@@ -64,6 +64,13 @@
           'include',
         ],
       },
+      'conditions': [
+       ['dart_want_separate_host_toolset==1', {
+              'toolsets': ['host'],
+            }, {
+              'toolsets': ['target'],
+            }],
+      ],
     },
   ],
 }