Put in place some minimum browser versions

These are the values we use when `-sLEGACY_VM_SUPPORT` is set or when
the users set one of the `-sMIN_XX_VERSION` settings to zero.  Zero has
always implied "oldest supported version".

The upside of this is that when babel is run now with
`-sLEGACY_VM_SUPPORT` we get a babel config file that looks like this:

```
{
  "sourceType": "script",
  "targets": {
    "chrome": "45",
    "firefox": "34",
    "safari": "9.0.0",
    "node": "10.19.0"
  }
}
```

As a followup I think we can remove any code blocks that are only active
for versions older than these.

tools/building.py

@@ -539,6 +539,7 @@ def transpile(filename):
   config_json = json.dumps(config, indent=2)
   outfile = shared.get_temp_files().get('babel.js').name
   config_file = shared.get_temp_files().get('babel_config.json').name
+  logger.debug(config_json)
   utils.write_file(config_file, config_json)
   cmd = shared.get_npm_cmd('babel') + [filename, '-o', outfile, '--presets', '@babel/preset-env', '--config-file', config_file]
   check_call(cmd, cwd=path_from_root())

tools/feature_matrix.py

@@ -15,6 +15,21 @@
 
 UNSUPPORTED = 0x7FFFFFFF
 
+# Oldest support browser versions.  These have been set somewhat
+# arbitrarily for now, based on:
+#  https://caniuse.com/mdn-javascript_builtins_object_assign
+#  -> FF:34 CHROME:45 SAFARI:9 NODE:4.0.0
+#  https://caniuse.com/promises
+#  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise
+#  -> FF:29 CHROME:33 SAFARI:8 NODE:0.12.0
+# TODO(sbc): Design a of policy for manging these values.
+OLDEST_SUPPORTED_CHROME = 45
+OLDEST_SUPPORTED_FIREFOX = 34
+OLDEST_SUPPORTED_SAFARI = 90000
+# 10.19.0 is the oldest version of node that we do any testing with.
+# Keep this in sync with the test-node-compat in .circleci/config.yml.
+OLDEST_SUPPORTED_NODE = 101900
+
 
 class Feature(IntEnum):
   NON_TRAPPING_FPTOINT = auto()

tools/link.py

@@ -621,6 +621,26 @@ def set_max_memory():
     exit_with_error('MAXIMUM_MEMORY cannot be less than INITIAL_MEMORY')
 
 
+def check_browser_versions():
+  settings_map = {
+    'MIN_FIREFOX_VERSION': feature_matrix.OLDEST_SUPPORTED_FIREFOX,
+    'MIN_CHROME_VERSION': feature_matrix.OLDEST_SUPPORTED_CHROME,
+    'MIN_SAFARI_VERSION': feature_matrix.OLDEST_SUPPORTED_SAFARI,
+    'MIN_NODE_VERSION': feature_matrix.OLDEST_SUPPORTED_NODE,
+  }
+
+  if settings.LEGACY_VM_SUPPORT:
+    # Support all old browser versions
+    for key, oldest in settings_map.items():
+      default_setting(key, oldest)
+
+  for key, oldest in settings_map.items():
+    if settings[key] == 0:
+      settings[key] = oldest
+    if settings[key] < oldest:
+      exit_with_error(f'{key} older than {oldest} is not supported')
+
+
 @ToolchainProfiler.profile_block('linker_setup')
 def phase_linker_setup(options, state, newargs):
   system_libpath = '-L' + str(cache.get_lib_dir(absolute=True))
@@ -1091,27 +1111,16 @@ def phase_linker_setup(options, state, newargs):
     if settings.MINIMAL_RUNTIME and options.oformat == OFormat.HTML and not settings.PTHREADS:
       settings.EXPORT_READY_PROMISE = 0
 
-  if settings.LEGACY_VM_SUPPORT:
-    if settings.WASM2JS:
-      settings.POLYFILL_OLD_MATH_FUNCTIONS = 1
+  if settings.WASM2JS and settings.LEGACY_VM_SUPPORT:
+    settings.POLYFILL_OLD_MATH_FUNCTIONS = 1
 
-    # Support all old browser versions
-    settings.MIN_FIREFOX_VERSION = 0
-    settings.MIN_SAFARI_VERSION = 0
-    settings.MIN_CHROME_VERSION = 0
-    settings.MIN_NODE_VERSION = 0
+  check_browser_versions()
 
   if settings.MIN_CHROME_VERSION <= 37:
     settings.WORKAROUND_OLD_WEBGL_UNIFORM_UPLOAD_IGNORED_OFFSET_BUG = 1
 
-  # 10.19.0 is the oldest version of node that we do any testing with.
-  # Keep this in sync with the test-node-compat in .circleci/config.yml
-  # and MINIMUM_NODE_VERSION in tools/shared.py
-  if settings.MIN_NODE_VERSION:
-    if settings.MIN_NODE_VERSION < 101900:
-      exit_with_error('targeting node older than 10.19.00 is not supported')
-    if settings.MIN_NODE_VERSION >= 150000:
-      default_setting('NODEJS_CATCH_REJECTION', 0)
+  if settings.MIN_NODE_VERSION >= 150000:
+    default_setting('NODEJS_CATCH_REJECTION', 0)
 
   # Do not catch rejections or exits in modularize mode, as these options
   # are for use when running emscripten modules standalone