Avoid re-downloading files that are already downloaded (#460)

* Writes a .emsdk_version file to output directories using the name (which contains a version number and/or hash) once all installation steps have completed successfully.
* If that file exists, skip downloading/installation hooks.

test.py

@@ -6,6 +6,19 @@ import subprocess
 import sys
 import tempfile
 
+WINDOWS = sys.platform.startswith('win')
+MACOS = sys.platform == 'darwin'
+
+upstream_emcc = os.path.join('upstream', 'emscripten', 'emcc')
+fastcomp_emcc = os.path.join('fastcomp', 'emscripten', 'emcc')
+emsdk = './emsdk'
+if WINDOWS:
+  upstream_emcc += '.bat'
+  fastcomp_emcc += '.bat'
+  emsdk = 'emsdk.bat'
+else:
+  emsdk = './emsdk'
+
 # Utilities
 
 
@@ -71,6 +84,9 @@ print('test .emscripten contents (latest was installed/activated in test.sh)')
 assert 'fastcomp' not in open(os.path.expanduser('~/.emscripten')).read()
 assert 'upstream' in open(os.path.expanduser('~/.emscripten')).read()
 
+# Test we don't re-download unnecessarily
+checked_call_with_output(emsdk + ' install latest', expected='already installed', unexpected='Downloading:')
+
 print('building proper system libraries')
 
 
@@ -108,19 +124,6 @@ def run_emsdk(cmd):
   check_call([emsdk] + cmd)
 
 
-WINDOWS = sys.platform.startswith('win')
-MACOS = sys.platform == 'darwin'
-
-upstream_emcc = os.path.join('upstream', 'emscripten', 'emcc')
-fastcomp_emcc = os.path.join('fastcomp', 'emscripten', 'emcc')
-emsdk = './emsdk'
-if WINDOWS:
-  upstream_emcc += '.bat'
-  fastcomp_emcc += '.bat'
-  emsdk = 'emsdk.bat'
-else:
-  emsdk = './emsdk'
-
 test_lib_building(upstream_emcc, use_asmjs_optimizer=True)
 
 print('update')