Generate AST nodes for field initializers.

dart/frog/leg/tree/nodes.dart

Index: dart/frog/leg/tree/nodes.dart
diff --git a/dart/frog/leg/tree/nodes.dart b/dart/frog/leg/tree/nodes.dart
index ef346e6501da77921a6d5b049f6aaccb23306ff1..82eaeccd5812d4b087729b442c257b847d01a481 100644
--- a/dart/frog/leg/tree/nodes.dart
+++ b/dart/frog/leg/tree/nodes.dart
@@ -536,7 +536,8 @@ class FunctionExpression extends Expression {
   }
 
   Token getBeginToken() {
-    return firstBeginToken(returnType, name);
+    if (returnType === null) return firstBeginToken(name, parameters);
+    return returnType.getBeginToken();
   }
 
   Token getEndToken() {
@@ -708,7 +709,10 @@ class Return extends Statement {
 
   Token getBeginToken() => beginToken;
 
-  Token getEndToken() => endToken;
+  Token getEndToken() {
+    if (endToken === null) return expression.getEndToken();
+    return endToken;
+  }
 }
 
 class ExpressionStatement extends Statement {