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

runtime/vm/kernel_binary_flowgraph.h

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef RUNTIME_VM_KERNEL_BINARY_FLOWGRAPH_H_
 #define RUNTIME_VM_KERNEL_BINARY_FLOWGRAPH_H_
 
 #if !defined(DART_PRECOMPILED_RUNTIME)
 
 #include <map>
@@ skipping 638 matching lines @@
   friend class VariableDeclarationHelper;
   friend class FieldHelper;
   friend class ProcedureHelper;
   friend class ClassHelper;
   friend class LibraryHelper;
   friend class ConstructorHelper;
   friend class SimpleExpressionConverter;
   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.
 // One can then for instance read the field from the call-site (and remember to
 // call SetAt to inform this helper class), and then use this to read more.
 // "Dumb" fields are stored (e.g. integers) and can be fetched from this class.
 // If asked to read a "non-dumb" field (e.g. an expression) it will be skipped.
 class FunctionNodeHelper {
  public:
   enum Fields {
@@ skipping 262 matching lines @@
         }
         if (++next_read_ == field) return;
       }
       case kType:
         builder_->SkipDartType();  // read type.
         if (++next_read_ == field) return;
       case kInitializer:
         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.
         }
         if (++next_read_ == field) return;
       case kEnd:
         return;
     }
   }
 
   void SetNext(Fields field) { next_read_ = field; }
@@ skipping 557 matching lines @@
 
   NameIndex canonical_name_;
   StringIndex name_index_;
   intptr_t source_uri_index_;
 
  private:
   StreamingFlowGraphBuilder* builder_;
   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
 
 #endif  // !defined(DART_PRECOMPILED_RUNTIME)
 #endif  // RUNTIME_VM_KERNEL_BINARY_FLOWGRAPH_H_