Declare support for async 2.0.0. (#1709)

Author: nex3

lib/src/barback.dart

@@ -40,7 +40,7 @@ final pubConstraints = {
   "barback": new VersionConstraint.parse(">=0.15.0 <0.15.3") as VersionRange,
   "source_span": new VersionConstraint.parse(">=1.0.0 <2.0.0") as VersionRange,
   "stack_trace": new VersionConstraint.parse(">=0.9.1 <2.0.0") as VersionRange,
-  "async": new VersionConstraint.parse(">=1.8.0 <2.0.0") as VersionRange
+  "async": new VersionConstraint.parse(">=1.8.0 <3.0.0") as VersionRange
 };
 
 /// Converts [id] to a "package:" URI.

pubspec.yaml

@@ -8,7 +8,7 @@ dependencies:
   # will be hosted dependencies.
   analyzer: ">=0.25.0 <0.31.0"
   args: ">=0.13.5 <2.0.0"
-  async: "^1.12.0"
+  async: ">=1.12.0 <3.0.0"
   barback: "^0.15.2"
   bazel_worker: "^0.1.4"
   cli_util: "^0.1.0"