Rather than in implementation of the abstract method in the
expanded anonymous class.

This leads to more more efficient use of the constant pool,
code shapes more amenable to SAM inlining, and is compatible
with the old behaviour of `-Xexperimental` in Scala 2.11,
which ScalaJS now relies upon.

Manual test:

```
scala> :paste -raw
// Entering paste mode (ctrl-D to finish)

package p1; trait T { val x = 0; def apply(): Any }; class DelambdafyInline { def t: T = (() => "") }

// Exiting paste mode, now interpreting.

scala> :javap -c p1.DelambdafyInline
Compiled from "<pastie>"
public class p1.DelambdafyInline {
  public p1.T t();
    Code:
       0: new           #10                 // class p1/DelambdafyInline$$anonfun$t$1
       3: dup
       4: aload_0
       5: invokespecial scala#16                 // Method p1/DelambdafyInline$$anonfun$t$1."<init>":(Lp1/DelambdafyInline;)V
       8: areturn

  public final java.lang.Object p1$DelambdafyInline$$$anonfun$1();
    Code:
       0: ldc           scala#22                 // String
       2: areturn

  public p1.DelambdafyInline();
    Code:
       0: aload_0
       1: invokespecial scala#25                 // Method java/lang/Object."<init>":()V
       4: return
}

scala> :javap -c p1.DelambdafyInline$$anonfun$t$1
Compiled from "<pastie>"
public final class p1.DelambdafyInline$$anonfun$t$1 implements p1.T,scala.Serializable {
  public static final long serialVersionUID;

  public int x();
    Code:
       0: aload_0
       1: getfield      scala#25                 // Field x:I
       4: ireturn

  public void p1$T$_setter_$x_$eq(int);
    Code:
       0: aload_0
       1: iload_1
       2: putfield      scala#25                 // Field x:I
       5: return

  public final java.lang.Object apply();
    Code:
       0: aload_0
       1: getfield      scala#34                 // Field $outer:Lp1/DelambdafyInline;
       4: invokevirtual scala#37                 // Method p1/DelambdafyInline.p1$DelambdafyInline$$$anonfun$1:()Ljava/lang/Object;
       7: areturn

  public p1.DelambdafyInline$$anonfun$t$1(p1.DelambdafyInline);
    Code:
       0: aload_1
       1: ifnonnull     6
       4: aconst_null
       5: athrow
       6: aload_0
       7: aload_1
       8: putfield      scala#34                 // Field $outer:Lp1/DelambdafyInline;
      11: aload_0
      12: invokespecial scala#42                 // Method java/lang/Object."<init>":()V
      15: aload_0
      16: invokespecial scala#45                 // Method p1/T.$init$:()V
      19: return
}

scala> :quit
```

Adriaan is to `git blame` for `reflection-mem-typecheck.scala`.