Implement initializer asserts.

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/body_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
index 904cd8084b9b063e179bce12afe3d3fd4cf17a4f..57af8bc55678bf9041bb2949c59a9a95c09b34ed 100644
--- a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
@@ -7,7 +7,8 @@ library fasta.body_builder;
 import '../fasta_codes.dart'
     show FastaMessage, codeExpectedButGot, codeExpectedFunctionBody;
 
-import '../parser/parser.dart' show FormalParameterType, MemberKind, optional;
+import '../parser/parser.dart'
+    show Assert, FormalParameterType, MemberKind, optional;
 
 import '../parser/identifier_context.dart' show IdentifierContext;
 
@@ -2449,12 +2450,42 @@ class BodyBuilder extends ScopeListener<JumpTarget> implements BuilderHelper {
   }
 
   @override
-  void handleAssertStatement(Token assertKeyword, Token leftParenthesis,
+  void beginAssert(Token assertKeyword, Assert kind) {
+    debugEvent("beginAssert");
+    inInitializer = false;

[kasperl, 2017/06/08 10:23:14]

Add a comment to explain why you have to turn this off. When is the previous
value restored? In endInitializer?

[ahe, 2017/06/08 11:04:33]

This value is never restored. I've added a comment here and on the declaration
of inInitializer.

+  }
+
+  @override
+  void endAssert(Token assertKeyword, Assert kind, Token leftParenthesis,
       Token commaToken, Token rightParenthesis, Token semicolonToken) {
-    debugEvent("AssertStatement");
+    debugEvent("Assert");
     Expression message = popForValueIfNotNull(commaToken);
     Expression condition = popForValue();
-    push(new AssertStatement(condition, message));
+    AssertStatement statement = new AssertStatement(condition, message);
+    switch (kind) {
+      case Assert.Statement:
+        push(statement);
+        break;
+
+      case Assert.Expression:
+        push(buildCompileTimeError("`assert` can't be used as an expression."));
+        break;
+
+      case Assert.Initializer:
+        push(buildAssertInitializer(statement));
+        break;
+    }
+  }
+
+  Initializer buildAssertInitializer(AssertStatement statement) {
+    return new LocalInitializer(new VariableDeclaration.forValue(

[kasperl, 2017/06/08 10:23:14]

It would be useful if you could explain the structure of the initializer you're
building here in a comment.

[ahe, 2017/06/08 11:04:33]

Done.

+        buildMethodInvocation(
+            new FunctionExpression(new FunctionNode(statement)),
+            callName,
+            new Arguments.empty(),
+            statement.fileOffset,
+            isConstantExpression: true,
+            isImplicitCall: true)));
   }
 
   @override