Starting work on full GN build

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
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+# Copyright (c) 2016, 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.
+
+# This target will be built if no target is specified when invoking ninja.
+group("default") {
+  deps = [
+    ":runtime",
+  ]
+}
+
+group("runtime") {
+  deps = [
+    "//runtime/bin:dart",
+    "//runtime/bin:dart_bootstrap($host_toolchain)",
+    "//runtime/bin:run_vm_tests",
+    "//runtime/bin:process_test",
+    "//runtime/bin:test_extension",
+    "//runtime/bin:sample_extension",
+  ]
+}