Revert "Roll dart to 29d6e963702fcbb0ed0d5ca67d6b5f977ac0c328 (#5083)" (flutter/engine#5085)

This reverts commit efeed7c717.

DEPS

@@ -31,7 +31,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/master/DEPS.
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': '29d6e963702fcbb0ed0d5ca67d6b5f977ac0c328',
+  'dart_revision': 'fe606f890b0a311da802c78b0af414a3c2087a79',
 
   'dart_args_tag': '1.4.1',
   'dart_async_tag': '2.0.6',

engine/src/flutter/tools/licenses/pubspec.lock

@@ -1,5 +1,5 @@
 # Generated by pub
-# See https://www.dartlang.org/tools/pub/glossary#lockfile
+# See http://pub.dartlang.org/doc/glossary.html#lockfile
 packages:
   archive:
     dependency: "direct main"
@@ -58,4 +58,4 @@ packages:
     source: hosted
     version: "1.1.5"
 sdks:
-  dart: ">=1.21.0 <=2.0.0-edge.c080951d45e79cd25df98036c4be835b284a269c"
+  dart: ">=1.21.0 <=2.0.0-dev.20.0"