Add support for configuration-specific imports to the VM.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index a33c4cce1f4400cb53017017785efef1431204ac..9062b139e8db772e2f69fafe59a75ffa37bea670 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -46,6 +46,8 @@ DEFINE_FLAG(bool, load_deferred_eagerly, false,
 DEFINE_FLAG(bool, trace_parser, false, "Trace parser operations.");
 DEFINE_FLAG(bool, warn_mixin_typedef, true, "Warning on legacy mixin typedef.");
 DEFINE_FLAG(bool, link_natives_lazily, false, "Link native calls lazily");
+DEFINE_FLAG(bool, conditional_directives, false,
+    "Enable conditional directives");
 
 DECLARE_FLAG(bool, lazy_dispatchers);
 DECLARE_FLAG(bool, load_deferred_eagerly);
@@ -5832,6 +5834,48 @@ void Parser::ParseLibraryImportExport(intptr_t metadata_pos) {
   ConsumeToken();
   CheckToken(Token::kSTRING, "library url expected");
   AstNode* url_literal = ParseStringLiteral(false);
+  if (FLAG_conditional_directives) {
+    bool condition_triggered = false;
+    while (CurrentToken() == Token::kIF) {
+      // Conditional import: if (env == val) uri.
+      ConsumeToken();
+      ExpectToken(Token::kLPAREN);
+      // Parse dotted name.
+      GrowableHandlePtrArray<const String> pieces(Z, 3);
+      pieces.Add(*ExpectIdentifier("identifier expected"));
+      while (CurrentToken() == Token::kPERIOD) {
+        pieces.Add(Symbols::Dot());
+        ConsumeToken();
+        pieces.Add(*ExpectIdentifier("identifier expected"));
+      }
+      String& key = String::ZoneHandle(Z);

[Ivan Posva, 2015/12/17 06:29:43]

Why is this not a const String&? Also you do not need it as a ZoneHandle here
either.

[floitsch, 2015/12/17 17:08:43]

Done.

+      key = Symbols::FromConcatAll(pieces);

[Ivan Posva, 2015/12/17 06:29:43]

The key does not need to be a Symbol and should only be allocated/concatenated
if it is actually going to be used below in the lookup.

[floitsch, 2015/12/17 17:08:43]

There isn't a good way to concatenate strings except by going through symbols. I
copied a TODO from Srdjan (from report.cc) where the same pattern was used.

Moved it below, so it's only computed, when needed.

+      AstNode* valueNode = NULL;
+      if (CurrentToken() == Token::kEQ) {
+        ConsumeToken();
+        CheckToken(Token::kSTRING, "string literal expected");
+        valueNode = ParseStringLiteral(false);
+      }
+      ExpectToken(Token::kRPAREN);
+      CheckToken(Token::kSTRING, "library url expected");
+      AstNode* conditional_url_literal = ParseStringLiteral(false);
+
+      // If there was already a condition that triggered, don't try to match
+      // again.
+      if (condition_triggered) continue;

[Ivan Posva, 2015/12/17 06:29:43]

VM style is:
if (expr) {
  statements;
}

[floitsch, 2015/12/17 17:08:43]

Done.

+      // Check if this conditional line overrides the default import.
+      const String& value = valueNode == NULL

[Ivan Posva, 2015/12/17 06:29:43]

(valueNode == NULL)

[floitsch, 2015/12/17 17:08:43]

Done.

+          ? Symbols::True()
+          : String::Cast(valueNode->AsLiteralNode()->literal());

[Ivan Posva, 2015/12/17 06:29:43]

Please use the same assertions as below on line 5879 and 5880 to verify that we
have the appropriate literal.

[floitsch, 2015/12/17 17:08:43]

Done.

+      // Call the embedder to supply us with the environment.
+      const String& env_value =
+          String::Handle(Api::CallEnvironmentCallback(isolate(), key));
+      if (!env_value.IsNull() && env_value.Equals(value)) {
+        condition_triggered = true;
+        url_literal = conditional_url_literal;
+      }
+    }
+  }
   ASSERT(url_literal->IsLiteralNode());
   ASSERT(url_literal->AsLiteralNode()->literal().IsString());
   const String& url = String::Cast(url_literal->AsLiteralNode()->literal());