[kernel] Use helper functions to minimize assumptions about dill layout.

runtime/vm/kernel_binary_flowgraph.h

Index: runtime/vm/kernel_binary_flowgraph.h
diff --git a/runtime/vm/kernel_binary_flowgraph.h b/runtime/vm/kernel_binary_flowgraph.h
index daf12245225fbe6557dff61427841f0bb4142a09..a9120ad8f7d3f570d65864d5bcf0e4f84461f7a4 100644
--- a/runtime/vm/kernel_binary_flowgraph.h
+++ b/runtime/vm/kernel_binary_flowgraph.h
@@ -656,6 +656,27 @@ class StreamingFlowGraphBuilder {
   friend class KernelReader;
 };
 
+// A helper class that saves the current reader position, goes to another reader
+// position, and upon destruction, resets to the original reader position.
+class AlternativeReadingScope {
+ public:
+  AlternativeReadingScope(Reader* reader, intptr_t new_position)
+      : reader_(reader), saved_offset_(reader_->offset()) {
+    reader_->set_offset(new_position);
+  }
+
+  explicit AlternativeReadingScope(Reader* reader)
+      : reader_(reader), saved_offset_(reader_->offset()) {}
+
+  ~AlternativeReadingScope() { reader_->set_offset(saved_offset_); }
+
+  intptr_t saved_offset() { return saved_offset_; }
+
+ private:
+  Reader* reader_;
+  intptr_t saved_offset_;
+};
+
 // Helper class that reads a kernel FunctionNode from binary.
 //
 // Use ReadUntilExcluding to read up to but not including a field.
@@ -938,16 +959,16 @@ class FieldHelper {
         if (builder_->ReadTag() == kSomething) {
           if (detect_function_literal_initializer &&
               builder_->PeekTag() == kFunctionExpression) {
-            has_function_literal_initializer_ = true;
-            intptr_t expr_offset = builder_->ReaderOffset();
+            AlternativeReadingScope alt(builder_->reader_);
             Tag tag = builder_->ReadTag();
             ASSERT(tag == kFunctionExpression);
-            tag = builder_->ReadTag();
-            ASSERT(tag == kFunctionNode);
-            function_literal_start_ = builder_->ReadPosition();
-            function_literal_end_ = builder_->ReadPosition();
 
-            builder_->SetOffset(expr_offset);
+            FunctionNodeHelper helper(builder_);
+            helper.ReadUntilIncluding(FunctionNodeHelper::kEndPosition);
+
+            has_function_literal_initializer_ = true;
+            function_literal_start_ = helper.position_;
+            function_literal_end_ = helper.end_position_;
           }
           builder_->SkipExpression();  // read initializer.
         }
@@ -1525,27 +1546,6 @@ class LibraryHelper {
   intptr_t next_read_;
 };
 
-// A helper class that saves the current reader position, goes to another reader
-// position, and upon destruction, resets to the original reader position.
-class AlternativeReadingScope {
- public:
-  AlternativeReadingScope(Reader* reader, intptr_t new_position)
-      : reader_(reader), saved_offset_(reader_->offset()) {
-    reader_->set_offset(new_position);
-  }
-
-  explicit AlternativeReadingScope(Reader* reader)
-      : reader_(reader), saved_offset_(reader_->offset()) {}
-
-  ~AlternativeReadingScope() { reader_->set_offset(saved_offset_); }
-
-  intptr_t saved_offset() { return saved_offset_; }
-
- private:
-  Reader* reader_;
-  intptr_t saved_offset_;
-};
-
 }  // namespace kernel
 }  // namespace dart