Adding test blocks into the playground

[AnmAtAnm]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide

The details

Resolves

#1626

Adds a number of test blocks to the playground for testing block configurations (and edge cases) not covered by the standard blocks.

Proposed Changes

Adds a new toolbox option to the playground, which includes only test blocks.
Does not change the blocks shown in other toolbox configurations.

Blocks include:

• Empty block (with and without a mutator)
• Various configurations of number blocks.
• Long drop-down fields and image drop-down fields.
• Angle and date fields.
• Image fields, including networked images and mis-specified images.

Reason for Changes

Provides an easy way to test these cases, encouraging more regular spot testing.

Test Coverage

Test blocks in this PR reference some network images that are not yet online. However, I tested them with URLs from the android project before setting them up with blockly-demo URL.

Tested on:

• Desktop Chrome

Additional Information

[picklesrus]

can these use the media url thingy?

[picklesrus]

Doh. never mind. I now realize getting them off the network was the whole point of this block :)

[picklesrus]

Did you make the images?

Please ignore my other comment :)

[AnmAtAnm]

I made the letter images. The LED icon I pulled as creative commons.

Imagess on Android repo:

• https://github.com/google/blockly-android/tree/gh-pages/
• https://github.com/google/blockly-android/tree/develop/blocklydemo/src/main/assets/images

Block definitions:

• https://github.com/google/blockly-android/blob/develop/blocklydemo/src/main/assets/sample_sections/mock_block_definitions.json
• https://github.com/google/blockly-android/blob/develop/blocklylib-core/src/main/assets/default/test_blocks.json

[picklesrus]

LGTM

[AnmAtAnm]

Travis failure unrelated to code:

{ Error: Couldn't connect to selenium server
    at getHTML("#closureTestRunnerLog") - test_runner.js:17:2
  details: undefined,
  message: 'connect ECONNREFUSED 127.0.0.1:4444',
  type: 'RuntimeError',
  seleniumStack: 
   { status: -1,
     type: 'ECONNREFUSED',
     orgStatusMessage: 'connect ECONNREFUSED 127.0.0.1:4444',
     message: 'Couldn\'t connect to selenium server' } }