Update package prep script to do async process execution and emit output as it happens. (flutter#13918)

 - Switches to async process execution, which now shows output as it happens instead of in chunks when the process completes
-  Now uses ProcessManager so that it may be mocked for the test.
- Adds in the download and install of mingit on Windows.
- Updated package dependencies because of added dependency on process package.

Author: gspencergoog

dev/automated_tests/pubspec.yaml

@@ -7,7 +7,7 @@ dependencies:
 
   args: 0.13.7 # TRANSITIVE DEPENDENCY
   async: 1.13.3 # TRANSITIVE DEPENDENCY
-  barback: 0.15.2+13 # TRANSITIVE DEPENDENCY
+  barback: 0.15.2+14 # TRANSITIVE DEPENDENCY
   boolean_selector: 1.0.2 # TRANSITIVE DEPENDENCY
   charcode: 1.1.1 # TRANSITIVE DEPENDENCY
   cli_util: 0.1.2+1 # TRANSITIVE DEPENDENCY
@@ -16,7 +16,7 @@ dependencies:
   crypto: 2.0.2+1 # TRANSITIVE DEPENDENCY
   csslib: 0.14.1 # TRANSITIVE DEPENDENCY
   glob: 1.1.5 # TRANSITIVE DEPENDENCY
-  html: 0.13.2+1 # TRANSITIVE DEPENDENCY
+  html: 0.13.2+2 # TRANSITIVE DEPENDENCY
   http: 0.11.3+14 # TRANSITIVE DEPENDENCY
   http_multi_server: 2.0.4 # TRANSITIVE DEPENDENCY
   http_parser: 3.1.1 # TRANSITIVE DEPENDENCY

dev/benchmarks/complex_layout/pubspec.yaml

@@ -19,7 +19,7 @@ dev_dependencies:
 
   args: 0.13.7 # TRANSITIVE DEPENDENCY
   async: 1.13.3 # TRANSITIVE DEPENDENCY
-  barback: 0.15.2+13 # TRANSITIVE DEPENDENCY
+  barback: 0.15.2+14 # TRANSITIVE DEPENDENCY
   boolean_selector: 1.0.2 # TRANSITIVE DEPENDENCY
   charcode: 1.1.1 # TRANSITIVE DEPENDENCY
   cli_util: 0.1.2+1 # TRANSITIVE DEPENDENCY
@@ -29,7 +29,7 @@ dev_dependencies:
   csslib: 0.14.1 # TRANSITIVE DEPENDENCY
   file: 2.3.5 # TRANSITIVE DEPENDENCY
   glob: 1.1.5 # TRANSITIVE DEPENDENCY
-  html: 0.13.2+1 # TRANSITIVE DEPENDENCY
+  html: 0.13.2+2 # TRANSITIVE DEPENDENCY
   http: 0.11.3+14 # TRANSITIVE DEPENDENCY
   http_multi_server: 2.0.4 # TRANSITIVE DEPENDENCY
   http_parser: 3.1.1 # TRANSITIVE DEPENDENCY

dev/benchmarks/microbenchmarks/pubspec.yaml

@@ -11,7 +11,7 @@ dependencies:
 
   args: 0.13.7 # TRANSITIVE DEPENDENCY
   async: 1.13.3 # TRANSITIVE DEPENDENCY
-  barback: 0.15.2+13 # TRANSITIVE DEPENDENCY
+  barback: 0.15.2+14 # TRANSITIVE DEPENDENCY
   boolean_selector: 1.0.2 # TRANSITIVE DEPENDENCY
   charcode: 1.1.1 # TRANSITIVE DEPENDENCY
   cli_util: 0.1.2+1 # TRANSITIVE DEPENDENCY
@@ -21,7 +21,7 @@ dependencies:
   csslib: 0.14.1 # TRANSITIVE DEPENDENCY
   dart_style: 1.0.9 # TRANSITIVE DEPENDENCY
   glob: 1.1.5 # TRANSITIVE DEPENDENCY
-  html: 0.13.2+1 # TRANSITIVE DEPENDENCY
+  html: 0.13.2+2 # TRANSITIVE DEPENDENCY
   http: 0.11.3+14 # TRANSITIVE DEPENDENCY
   http_multi_server: 2.0.4 # TRANSITIVE DEPENDENCY
   http_parser: 3.1.1 # TRANSITIVE DEPENDENCY