Have npm run typings use built directory; fix typescript tests

* Modify scripts/gulpfiles/typings.js to write typings to BUILD_DIR.
* Modify tests/scripts/compile_typings.sh to check compilability of
  resulting output from BUILD_DIR.
* Rename checkin script to checkin:built, add a checkin:typings script
  to do the same for .d.ts files, and a new checkin script to do both.
* Have recompile run checkin:typings.

tests/scripts/compile_typings.sh

@@ -1,21 +1,21 @@
 #!/bin/bash
 
+# Location that npm run typings will write .d.ts files to.
+readonly BUILD_DIR='built'
+
 # ANSI colors
 BOLD_GREEN='\033[1;32m'
 BOLD_RED='\033[1;31m'
 ANSI_RESET='\033[0m'
 
-# Download TypeScript to obtain the compiler.
-echo "Downloading TypeScript"
-npm install typescript
-
 # Generate Blockly typings.
 echo "Generating Blockly typings"
 npm run typings
 
 # Use the TypeScript compiler to compile the generated typings.
 echo "Compiling typings"
-cd typings
+
+cd "${BUILD_DIR}"
 ../node_modules/.bin/tsc blockly.d.ts