Add pubspec.yaml files for each of the examples/ directories

so that pub get will create a packages/ directory for each
and users of packages/sky/sky_tool can run these examples
from sky_sdk as the instructions say to.

TBR=ianh@google.com
https://github.com/domokit/mojo/issues/128

Review URL: https://codereview.chromium.org/1110283002

examples/README.md

 Sky Examples
 ============
 
-This directory contains several examples of using Sky.
+This directory contains several examples of using Sky.  Each of these is an
+individual Dart application package.  If you wish to run them with `sky_tool`
+then you will want to run `pub get` inside their directory before running
+`./packages/sky/sky_tool start`.
 
 1. *Hello, world.* The [hello world app](hello_world) is a basic app that shows
    the text "hello, world."

examples/color/pubspec.yaml

+name: color
+dependencies:
+ sky: any

examples/hello_world/pubspec.yaml

+name: hello_world
+dependencies:
+ sky: any

examples/raw/pubspec.yaml

+name: raw
+dependencies:
+ sky: any

examples/widgets/pubspec.yaml

+name: widgets
+dependencies:
+ sky: any

sdk/README.md

@@ -131,13 +131,23 @@ Running a Sky application
 -------------------------
 
 The `sky` pub package includes a `sky_tool` script to assist in running
-Sky applications inside the `SkyDemo.apk` harness.
+Sky applications inside the `SkyDemo.apk` harness.  The sky_tool script expects
+to be run from the root directory of your application pub package.  To run
+one of the examples in this SDK, try:
 
-1. ``packages/sky/sky_tool start --install examples/stocks/main.sky``
+1. ``cd examples/stocks``
+
+2. ``pub get`` to set up a copy of the sky package in the app directory.
+
+3. ``./packages/sky/sky_tool start --install``
    The --install flag is only necessary to install SkyDemo.apk if not already
-   installed from the Google Play store.
+   installed on the device.
 
-2.  Use ``adb logcat`` to view any errors or Dart print() output from the app.
+4. Use ``adb logcat`` to view any errors or Dart print() output from the app.
+   ``adb logcat -s chromium`` can be used to filter only adb messages from
+   `SkyDemo.apk` (which for
+   [legacy reasons](https://github.com/domokit/mojo/issues/129) still uses the
+   android log tag 'chromium').
 
 Measuring Performance
 ---------------------