move collection to dev dependency (#9613)

packages/flutter_tools/pubspec.yaml

@@ -10,7 +10,6 @@ environment:
 dependencies:
   archive: ^1.0.20
   args: ^0.13.4
-  collection: '>=1.9.1 <2.0.0'
   coverage: ^0.8.0
   crypto: '>=1.1.1 <3.0.0'
   file: 2.3.2
@@ -42,6 +41,7 @@ dependencies:
   front_end: any
 
 dev_dependencies:
+  collection: '>=1.9.1 <2.0.0'
   mockito: ^2.0.2
 
 # Exclude this package from the hosted API docs.