Add IR nodes needed for deferred loading.

pkg/kernel/binary.md

 This file describes the binary format of Dart Kernel.
 
 Notation
 --------
 Bitmasks are described with the syntax:
 ```scala
     Byte flags (flag1, flag2, ..., flagN)
 ```
 where 'flag<N>' is the N-th least significant bit,
 (so flag1 is the least significant bit).
@@ skipping 157 matching lines @@
   Byte flags (isExternal);
   StringReference name;
   // A URI from which the library was created.  The URI has the dart, package,
   // file, or app scheme.  For file URIs, the path is an absolute path to the
   // .dart file from which the library was created.  For app URIs, the path is
   // relative to an application root that was specified when the binary was
   // generated.
   StringReference importUri;
   // An absolute path URI to the .dart file from which the library was created.
   UriReference fileUri;
+  List<DeferredImport> deferredImports;
   List<Class> classes;
   List<Field> fields;
   List<Procedure> procedures;
 }
 
+type DeferredImport {
+  LibraryReference importedLibrary;
+  StringReference name;
+}
+
+type DeferredImportReference {
+  // Index into deferredImports in the enclosing Library.
+  UInt index;
+}
+
 abstract type Node {
   Byte tag;
 }
 
 // A class can be represented at one of three levels: type, hierarchy, or body.
 //
 // If the enclosing library is external, a class is either at type or
 // hierarchy level, depending on its isTypeLevel flag.
 // If the enclosing library is not external, a class is always at body level.
 //
@@ skipping 464 matching lines @@
   Byte tag = 52;
   FunctionNode function;
 }
 
 type Let extends Expression {
   Byte tag = 53;
   VariableDeclaration variable;
   Expression body;
 }
 
+type LoadLibrary extends Expression {
+  Byte tag = 14;
+  DeferredImportReference import;
+}
+
+type CheckLibraryIsLoaded extends Expression {
+  Byte tag = 13;
+  DeferredImportReference import;
+}
+
 abstract type Statement extends Node {}
 
 type InvalidStatement extends Statement {
   Byte tag = 60;
 }
 
 type ExpressionStatement extends Statement {
   Byte tag = 61;
   Expression expression;
 }
@@ skipping 240 matching lines @@
 
 /* enum BaseClassKind { None, Exact, Subclass, Subtype, } */
 
 type InferredValue {
   ClassReference baseClass; // May be NullReference if kind = None.
   Byte kind; // Index into BaseClassKind.
   Byte valueBits; // See lib/type_propagation/type_propagation.dart
 }
 
 ```