Support mixins when loading from .dill

pkg/compiler/lib/src/kernel/element_map.dart

Index: pkg/compiler/lib/src/kernel/element_map.dart
diff --git a/pkg/compiler/lib/src/kernel/element_map.dart b/pkg/compiler/lib/src/kernel/element_map.dart
index f66804a9529e58c258495347c158cdbcce685db9..57e9af31c0936b7f2eaf1874c7c241546b6e72b4 100644
--- a/pkg/compiler/lib/src/kernel/element_map.dart
+++ b/pkg/compiler/lib/src/kernel/element_map.dart
@@ -53,6 +53,23 @@ abstract class KernelToElementMap {
   /// constructor [node].
   ConstructorEntity getConstructor(ir.Member node);
 
+  /// Returns the [ConstructorEntity] corresponding to a super initializer
+  /// in [constructor] targeting [target].

[Siggi Cherem (dart-lang), 2017/05/17 16:50:32]

nit: consider rephrasing this a bit and include below a bit more details on what
the result is (in addition to what the "target" is).

In particular, I was a bit confused by "targeting [target]": IIUC, the result
matches [target] when [target] corresponds to the immediate superclass, but in
dart2js we need the entity for the mixin application instead. Correct?

Here is a possible tweak/suggestion:
 
Returns the [ConstructorEntity] corresponding to a super initializer in
[constructor].

The IR resolves super initializers to a [target] up in the type hierarchy. Most
of the time, the result of this function will be the entity corresponding to
that target. In the presence of unnamed mixins, this function returns an entity
for an intermediate synthetic constructor that kernel doesn't explicitly
represent. 

For example:
     class M {}
     class C extends Object with M {}

Kernel will say that C()'s super initializer resolves to Object(), but this
function will return an entity representing the unnamed mixin application
"Object+M"'s constructor.

[Johnni Winther, 2017/05/18 08:57:03]

Done.

+  ///
+  /// Depending on IR encoding of mixins, [target] might be the constructor of
+  /// the first non-mixin-application superclass of the class of [constructor].
+  ///
+  /// For instance
+  ///
+  ///     class M {}
+  ///     class C extends Object with M {}
+  ///
+  /// The default constructor of `C` might have the no-name constructor of
+  /// `Object` as target instead of the no-name constructor of the unnamed
+  /// mixin application `Object+M`.
+  ConstructorEntity getSuperConstructor(
+      ir.Constructor constructor, ir.Member target);
+
   /// Returns the [MemberEntity] corresponding to the member [node].
   MemberEntity getMember(ir.Member node);