[dart2js] Canonicalize empty collections in FunctionType

This reduces the number of _CompactLinkedHashSet objects by ~15% and
the number of _GrowableList objects by ~10% in Phase1.

Change-Id: I88ebe1b1c842977214985e23a1f3672ad43bdfdc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155826
Reviewed-by: Joshua Litt <joshualitt@google.com>
Commit-Queue: Stephen Adams <sra@google.com>

Author: rakudrama

pkg/compiler/lib/src/elements/types.dart

@@ -651,25 +651,50 @@ class FunctionType extends DartType {
 
   final List<FunctionTypeVariable> typeVariables;
 
-  FunctionType._(
+  FunctionType._allocate(
       this.returnType,
       this.parameterTypes,
       this.optionalParameterTypes,
       this.namedParameters,
       this.requiredNamedParameters,
       this.namedParameterTypes,
       this.typeVariables) {
-    assert(returnType != null, "Invalid return type in $this.");
-    assert(!parameterTypes.contains(null), "Invalid parameter types in $this.");
+    assert(returnType != null, 'Invalid return type in $this.');
+    assert(!parameterTypes.contains(null), 'Invalid parameter types in $this.');
     assert(!optionalParameterTypes.contains(null),
-        "Invalid optional parameter types in $this.");
+        'Invalid optional parameter types in $this.');
     assert(
-        !namedParameters.contains(null), "Invalid named parameters in $this.");
+        !namedParameters.contains(null), 'Invalid named parameters in $this.');
     assert(!requiredNamedParameters.contains(null),
-        "Invalid required named parameters in $this.");
+        'Invalid required named parameters in $this.');
     assert(!namedParameterTypes.contains(null),
-        "Invalid named parameter types in $this.");
-    assert(!typeVariables.contains(null), "Invalid type variables in $this.");
+        'Invalid named parameter types in $this.');
+    assert(!typeVariables.contains(null), 'Invalid type variables in $this.');
+  }
+
+  factory FunctionType._(
+      DartType returnType,
+      List<DartType> parameterTypes,
+      List<DartType> optionalParameterTypes,
+      List<String> namedParameters,
+      Set<String> requiredNamedParameters,
+      List<DartType> namedParameterTypes,
+      List<FunctionTypeVariable> typeVariables) {
+    // Canonicalize empty collections to constants to save storage.
+    if (parameterTypes.isEmpty) parameterTypes = const [];
+    if (optionalParameterTypes.isEmpty) optionalParameterTypes = const [];
+    if (namedParameterTypes.isEmpty) namedParameterTypes = const [];
+    if (requiredNamedParameters.isEmpty) requiredNamedParameters = const {};
+    if (typeVariables.isEmpty) typeVariables = const [];
+
+    return FunctionType._allocate(
+        returnType,
+        parameterTypes,
+        optionalParameterTypes,
+        namedParameters,
+        requiredNamedParameters,
+        namedParameterTypes,
+        typeVariables);
   }
 
   factory FunctionType._readFromDataSource(