Add --dart-flags and --disable-dart command line switches.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index bb3726c6c50d49eea1a133c94535b8638d393be8..f6a16cb910325529eb4b6d5bf2ea8ac2ef696907 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -623,6 +623,9 @@
     # https://bugs.webkit.org/show_bug.cgi?id=68463.
     'build_webkit_exes_from_webkit_gyp%': 1,
 
+    # Disable Dart by default.
+    'enable_dart%': 0,
+
     'conditions': [
       # Used to disable Native Client at compile time, for platforms where it
       # isn't supported (ARM)
@@ -1170,6 +1173,9 @@
           'ENABLE_WEB_INTENTS=1',
         ],
       }],
+      ['enable_dart==1', {
+        'defines': ['WEBKIT_USING_DART=1'],
+      }],
     ],  # conditions for 'target_defaults'
     'target_conditions': [
       ['enable_wexit_time_destructors==1', {