Dart2 version updates (#1708)

* fix fixture pubspecs

* bump build_web_compilers test dep

Author: jakemac53

build_modules/test/fixtures/a/pubspec.yaml

@@ -1,5 +1,8 @@
 name: a
 
+environment:
+  sdk: ">=1.0.0 <3.0.0"
+
 dependencies:
   b:
     path: ../b

build_modules/test/fixtures/b/pubspec.yaml

@@ -1,5 +1,8 @@
 name: b
 
+environment:
+  sdk: ">=1.0.0 <3.0.0"
+
 dependencies:
   a:
     path: ../a

build_web_compilers/pubspec.yaml

@@ -29,7 +29,7 @@ dependencies:
 
 dev_dependencies:
   build_test: ^0.10.0
-  test: ^0.12.24
+  test: ^1.0.0
   a:
     path: test/fixtures/a
   b:

build_web_compilers/test/fixtures/a/pubspec.yaml

@@ -1,5 +1,8 @@
 name: a
 
+environment:
+  sdk: ">=1.0.0 <3.0.0"
+
 dependencies:
   b:
     path: ../b

build_web_compilers/test/fixtures/b/pubspec.yaml

@@ -1,5 +1,8 @@
 name: b
 
+environment:
+  sdk: ">=1.0.0 <3.0.0"
+
 dependencies:
   a:
     path: ../a