Rename some ClassObject/Provider-related methods

The terminology "field" is preferred over "key" for the components of a struct or struct-like object.

RELNOTES: None
PiperOrigin-RevId: 180269374
diff --git a/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleClassFunctionsTest.java b/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleClassFunctionsTest.java
index 87561bd..930ba25 100644
--- a/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleClassFunctionsTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleClassFunctionsTest.java
@@ -1051,7 +1051,7 @@
         "y = struct(c = 1, d = 2)",
         "z = x + y\n");
     Info z = (Info) lookup("z");
-    assertThat(z.getKeys()).isEqualTo(ImmutableSet.of("a", "b", "c", "d"));
+    assertThat(z.getFieldNames()).isEqualTo(ImmutableSet.of("a", "b", "c", "d"));
   }
 
   @Test
diff --git a/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleImplementationFunctionsTest.java b/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleImplementationFunctionsTest.java
index 8dfb22d..877bcf8 100644
--- a/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleImplementationFunctionsTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleImplementationFunctionsTest.java
@@ -1186,7 +1186,7 @@
       getConfiguredTarget("//test:my_rule");
       fail();
     } catch (AssertionError expected) {
-      assertThat(expected).hasMessageThat().contains("Invalid key for default provider: foo");
+      assertThat(expected).hasMessageThat().contains("Invalid field for default provider: foo");
     }
   }